Prealgebra · Lesson 9.1

Percents as Fractions

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Percent means per hundred, from the Latin per centum. So 73% is literally 73100, a fraction whose denominator is 100. The % sign is just a stand-in for that /100.

Percent means per hundred 100 equal squares, 73 shaded 73% = 73 100 = 0.73
The word percent just means per hundred, so the whole grid is split into exactly 100 equal squares and 73 of them are filled. That shaded amount is 73%, which is the same as saying 73 out of 100, the fraction 73100. And since a fraction over 100 reads straight off as a decimal, that same shaded patch is also 0.73. One amount, three names, 73% and 73100 and 0.73, and the rest of this lesson is just learning to slide between them.
Problem
Your phone storage is 80% full. Write 80% as a fraction in lowest terms.
Show a hint
  • The word percent means "per hundred," so put the number over 100. That gives you 80100, and now it is just a fraction waiting to be simplified.
  • Both 80 and 100 are divisible by 20. Divide the top by 20 and the bottom by 20 to land on the smallest equivalent fraction.
Show the full solution
80% means 80 per hundred, so 80%=80100. The greatest common factor of 80 and 100 is 20, so divide the top and the bottom by 20. 80100=80÷20100÷20=45 So the answer is 4/5. Reducing does not shrink the amount, it just names the same slice of storage with smaller numbers.
A percent is just a fraction over 10080%80100divide top andbottom by 204536%36100divide top and bottom by 4925
A percent slides straight onto a denominator of 100, so 80% becomes 80100. From there it reduces like any other fraction, dividing top and bottom by 20 to land on 45. The same move turns 36% into 36100, and dividing by 4 gives 925.
Problem
A bakery sold 250% as many sourdough loaves as last Saturday. Percents can exceed 100. Write 250% as a fraction in lowest terms.
Show a hint
  • A percent means that many hundredths, so 250% is the same as 250100. Going above 100 is allowed, it simply gives a fraction larger than 1.
  • Now reduce 250100. Both the top and bottom share a factor of 50, so divide each by 50.
Show the full solution
A percent is a number of hundredths, so 250%=250100. The greatest common factor of 250 and 100 is 50, so divide the top and the bottom by 50. 250100=250÷50100÷50=52 So 250% in lowest terms is 5/2. Going past 100 is fine, it just means more than one whole, and 52 is 2.5, the two and a half times the bakery sold.
Problem
A glacier retreated by 45% over a decade. A percent can be negative. Write 45% as a fraction in lowest terms.
Show a hint
  • Start from the meaning. 45% means 45 out of 100, so write it as 45100. The negative sign just rides along on top.
  • Now reduce 45100. Both 45 and 100 share a factor of 5, so divide each by 5 and keep the minus sign.
Show the full solution
45% means 45 out of 100, so 45%=45100. Both 45 and 100 are divisible by 5, and dividing gives 45÷5100÷5=920. Since 9 and 20 share no factor bigger than 1, that is lowest terms, so 9/20. The minus sign rides along on the numerator and changes nothing about how you reduce.
Problem
A coupon takes 38% off. Rewrite 38% as a decimal.
Show a hint
  • A percent is a count per hundred, so 38%=38100=38÷100. You are not taking the percent of any price yet, just rewriting the number itself.
  • Dividing by 100 slides the decimal point two places to the left. Picture 38 as 38. and hop that point left twice, filling the empty spot with a zero.
Show the full solution
38%=38100=38÷100, and dividing by 100 moves the decimal point two places to the left. Start at 38., hop the point left once to get 3.8, then left again to land at 0.38. So 38%=0.38.
Divide by 100, slide the point two places left 38% means 38 1 2 0.38 ÷ 100 6% means 6 1 2 0. 0 placeholder 0 6 ÷ 100
Dividing by 100 walks the decimal point two steps to the left. So 38% becomes 0.38, and 6% becomes 0.06, where a placeholder zero fills the tenths spot that the missing digit leaves empty.
Problem
A bridge sensor drifts by 0.4%. Write 0.4% as a decimal.
Show a hint
  • A percent means "divided by one hundred", so dividing by 100 slides the decimal point two places to the left. Start from 0.4 and move the point.
  • Moving the point in 0.4 two places left runs out of digits, so fill the empty spots with zeros. You get 0.40.040.004.
Show the full solution
A percent is a count out of one hundred, so 0.4% means 0.4100. Dividing by 100 slides the decimal point two places to the left. Starting at 0.4, one slide gives 0.04 and a second slide gives 0.004, with zeros filling the empty places. So 0.4%=0.004.
Problem
Your fitness app shows daily goal progress as the decimal 0.41. Write 0.41 as a percent. Give just the number.
Show a hint
  • A percent is a count out of one hundred, so to name a decimal as a percent you multiply it by 100. Multiplying by 100 slides the decimal point two places to the right.
  • Slide the point in 0.41 two places right. It moves past the 4 and past the 1, landing after the 1, which leaves 41.
Show the full solution
A decimal becomes a percent when you multiply it by 100, and multiplying by 100 just slides the decimal point two places to the right. Start with 0.41. Slide the point past the 4 to get 4.1, then past the 1 to get 41. So 0.41×100=41, which means 0.41=41%. The answer is 41.
Problem
A race lap timer reads 0.073 of the way through the final lap. Write 0.073 as a percent. Give just the number.
Show a hint
  • To go from a decimal to a percent, multiply by 100, which slides the decimal point two places to the right. Start by moving it from 0.073 one place, then a second place.
  • Sliding 0.073 two places right gives 007.3. Drop the leading zeros and you are left with a number that still has a digit after the point.
Show the full solution
To make a decimal into a percent, multiply by 100, which slides the decimal point two places to the right. From 0.073, one slide gives 0.73 and a second gives 7.3, so 0.073=7.3% and the number you type is 7.3. There were three digits after the point, so only two of them clear it and the last one stays behind in the tenths place. That is why the percent is itself a decimal.
Problem
A garden bed is 34 planted. A percent has a hidden denominator of 100. Write 34 as a percent. Give just the number.
Show a hint
  • You want the same fraction with a denominator of 100. Ask yourself what 4 must be multiplied by to reach 100, and remember to multiply the top by that same number.
  • Since 4×25=100, multiply top and bottom by 25. That gives 3×254×25=75100, and the numerator over 100 is the percent.
Show the full solution
Rewrite 34 as an equal fraction whose denominator is 100. Since 4×25=100, multiply the top and bottom by the same 25. 34=3×254×25=75100. A fraction over 100 is read directly as a percent, so 75100=75%. The percent is 75.
One number, three costumes over 100, simplify ↓ ↑ denominator 100 ↓ ÷ 100, point 2 left × 100, point 2 right ↑ top ÷ bottom → ← over a power of ten one number, three costumes Percent 75% Fraction 3 4 Decimal 0.75 75% = 3/4 = 0.75
Three corners, one value. Walk the left side and 75% becomes 75100=34, then back up by rewriting with denominator 100. Walk the right side and 75% becomes 0.75 by sliding the point two places left, then back up by sliding it two places right. Walk the bottom and 34=3÷4=0.75. A percent, a fraction, and a decimal are three forms of the same value, and these arrows carry you between any two.
Problem
A bakery scales a recipe to 85 of the original. Since 85>1, the percent lands above 100. Write 85 as a percent. Answer with just the number.
Show a hint
  • To get a denominator of 100, ask what turns 5 into 100. Since 5×20=100, multiply both the top and the bottom by 20.
  • Multiplying gives 8×205×20=160100. The numerator over 100 is the percent.
Show the full solution
Since 5×20=100, rewrite 85 with a denominator of 100 by multiplying the top and bottom by 20. 85=8×205×20=160100 A fraction over 100 reads straight off as a percent, so the answer is 160. Because 85 is bigger than one whole, the percent lands above 100, exactly as expected.
Problem
A recipe calls for 98 cups of oats. Since eighths do not divide 100 evenly, multiply by 100 and simplify. Write 98 as a percent. Give just the number.
Show a hint
  • To turn a fraction into a percent, multiply it by 100. So you want 98×100, which is the same as 9008.
  • Now simplify 9008 by dividing top and bottom by 4 to get 2252, then divide 225 by 2. It does not come out even, so expect a decimal ending in a half.
Show the full solution
A fraction becomes a percent when you multiply it by 100. 98×100=9×1008=9008 Divide the top and bottom by 4 to get 2252, and 225÷2=112.5. So the number is 112.5. It clears 100 because 98 is more than one whole, and it carries a .5 because halving an odd number leaves a half.
Problem
Room A chose pizza at 65%. Room B chose pizza at 35. Convert both to decimals and pick the larger share. Type 1 for Room A or 2 for Room B.
Show a hint
  • You cannot compare a percent and a fraction while they look different. Turn both into decimals first. For the percent, slide the point two places left. For the fraction, divide the top by the bottom.
  • Room A is 65%=0.65. Room B is 35=0.60. Now both are decimals, so just compare 0.65 with 0.60 and name the option that is bigger.
Show the full solution
Put both shares in decimal form. Room A is 65%, and dividing by 100 slides the point two places left, so 65%=0.65. Room B is 35, and 3÷5=0.60, which you can check with 35=610=0.60. Since 0.65>0.60, Room A has the larger share, option 1. Values in different forms cannot be judged by eye, so convert them to one shared form first.
Problem
A sales tax is 12.5%. Write 12.5% as a fraction in lowest terms.
Show a hint
  • Percent means per hundred, so start by writing 12.5% as 12.5100. A fraction in lowest terms cannot keep a decimal upstairs, so you need to clear that half.
  • Multiply the top and bottom by 10 to get rid of the decimal point. That turns 12.5100 into 1251000, and now both numbers are whole and ready to simplify.
Show the full solution
Percent means per hundred, so 12.5%=12.5100. Multiply the top and bottom by 10 to clear the decimal. 12.5100=12.5×10100×10=1251000 Both 125 and 1,000 share the factor 125, and dividing each by it gives 1/8. Twelve and a half percent being exactly one eighth makes sense, since 18 of a hundred is 12.5.

Practice these ideas

Practice
A survey found 55% of students bike to school. Write 55% as a fraction in lowest terms.
Show the solution
Percent means "per hundred," so 55% is 55 out of 100. 55%=55100 Now reduce. The numerator and denominator both share a factor of 5, so divide each by 5. 55100=55÷5100÷5=1120 Since 11 is prime and does not divide 20, the fraction is in lowest terms. 11/20
Practice
A pollster reports 24% of people surveyed had never tasted dark chocolate. Write 24% as a fraction in lowest terms.
Show the solution
The word percent means per hundred, so 24% is 24 out of 100. 24%=24100 Now simplify. The greatest factor shared by 24 and 100 is 4, so divide both the numerator and the denominator by 4. 24100=24÷4100÷4=625 Since 6 and 25 share no common factor larger than 1, this is fully reduced. 6/25
Practice
A loyalty card stamps your total at 320% of last month. Write 320% as a fraction in lowest terms.
Show the solution
Percent means per hundred, so 320%=320100. The greatest common factor of 320 and 100 is 20, so divide both parts by it. That gives 320÷20100÷20=165. Since 16 and 5 share no factor bigger than 1, this is lowest terms, and it is more than one whole because 16>5. 16/5
Practice
A deep freezer drifted 8% from its set point. Write 8% as a fraction in lowest terms.
Show the solution
Percent means per hundred, so the number sits over 100 and keeps its sign. That gives 8%=8100. The top and bottom share a common factor of 4, so divide both by 4. The numerator becomes 8÷4=2 and the denominator becomes 100÷4=25. Since 2 and 25 have no common factor left, the fraction is in lowest terms. So 8% equals 2/25.
Practice
A weather app says the chance of rain is 73%. Write 73% as a decimal.
Show the solution
A percent is just a number of hundredths, so 73% means 73100. Dividing by 100 slides the decimal point two places to the left, turning 73. into 0.73. So the decimal form is 0.73.
Practice
Write 108% as a decimal.
Show the solution
To write a percent as a decimal you divide by 100, which slides the point two places to the left. Begin with 108, really 108.0, and move the point two places left to land between the 1 and the first 0. So 108%=108100=1.08. The result is more than 1 because 108% is more than a whole.
Practice
A lab label reads 0.6% saline. Write 0.6% as a decimal.
Show the solution
A percent is just a number over 100, so 0.6% means 0.6100. Dividing by 100 slides the decimal point two places to the left. Starting from 0.6, one place left gives 0.06, and a second place gives 0.006, with placeholder zeros filling the gaps. So 0.6% as a decimal is 0.006.
Practice
A probe is filled to 0.47 of its tank. Write 0.47 as a percent. Give just the number.
Show the solution
To turn a decimal into a percent, multiply by 100, which slides the point two places to the right. So 0.47×100=47, and the number to post is 47. It also reads straight off, since 0.47=47100 is already 47 per hundred.
Practice
A grain of gold dust registers 0.025 of a carat. Write 0.025 as a percent. Give just the number.
Show the solution
To go from a decimal to a percent, multiply by 100, which slides the decimal point two places to the right. 0.025×100=2.5 Sliding the point from 0.025 two places to the right lands between the 2 and the 5, so the value is 2.5%. The number alone is 2.5.
Practice
A trail map marks 720 of its loops as beginner-friendly. Write 720 as a percent. Give just the number.
Show the solution
Multiplying 20 by 5 gives 100, so multiply the top by 5 as well to keep the same value. 720=7×520×5=35100 A fraction over 100 reads straight off as a percent, so the number is 35. Rescaling like this works whenever the denominator divides 100 evenly.
Practice
A juice bottle is 58 real fruit. Write 58 as a percent. Give just the number.
Show the solution
To turn a fraction into a percent, multiply it by 100. 58×100=5008=62.5 So 58 of the bottle is 62.5% fruit, and the number is 62.5.
Practice
Write 115 as a percent. Give just the number.
Show the solution
To write a fraction as a percent, multiply it by 100. That gives 115×100=1,1005, and 1,100÷5=220, so 115=220% and the number is 220. That checks out, since 115 is a bit more than 2 and 220% is a bit more than 200%.
Practice
A pool is 87.5% full. Write 87.5% as a fraction in lowest terms.
Show the solution
Percent means per hundred, so 87.5%=87.5100. Multiply the top and bottom by 10 to clear the decimal, which gives 8751000. Now reduce. Both numbers are divisible by 125, and 875÷125=7 while 1,000÷125=8, so the fraction becomes 78. Since 7 is prime and does not divide 8, this is lowest terms. 7/8
Practice
Option 1 is 920 and option 2 is 44%. Which value is larger? Type 1 or 2.
Show the solution
Put both into percent form so they can be compared head to head. For the fraction, scale 920 to a denominator of 100 by multiplying top and bottom by 5, which gives 45100=45%. Now line them up, 45% against 44%. Since 45%>44%, option 1 is the larger value. 1