Percent benchmark recall sprint

Use 10%, 25%, 50%, 20% benchmarks to compute percents mentally.

A fluency sprint: build the speed and accuracy you need to do this calculation instantly on test day.

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How to find benchmark percents in your head

Treat each benchmark percent as a simple fraction or a quick move of the decimal point.

Common slips: moving the decimal point the wrong number of places (the answer is then 10 times too big or too small), and finding the part that is left (100% minus the percent) instead of the part asked for. Sense check: 75% of a number must be more than half of it.

Worked examples

Example 1.
A desk lamp costs \(\$18.00\). What is 20% of that price?
Show answer and explanation
Answer:
C

20% of \(\$18.00\) \(= 20/100 \times 18 = \$3.60\). (Dividing by 10 instead of 100 (\(18 \times 20 \div 10\)) gives $36.00. Finding the price after taking 20% off (\(\$18.00 - \$3.60\)) gives $14.40. Dividing by 1000 instead of 100 gives $0.36.) 20% of the price is $3.60.

Example 2.
Of the 616 voters at the town election, 75% arrived before 9 a.m. How many voters is that?
Show answer and explanation
Answer:
B

75% of 616 \(= 75/100 \times 616 = 462\). (Finding the voters who did NOT arrive before 9 a.m. (\(616 - 462\)) gives 154. Subtracting 75 voters instead of 75% (\(616 - 75\)) gives 541. Reading 75% as 75 voters gives 75.) That is 462 voters.

Example 3.
Of the 650 campers at the summer camp, 90% arrived before 9 a.m. How many campers is that?
Show answer and explanation
Answer:
C

90% of 650 \(= 90/100 \times 650 = 585\). (Finding the campers who did NOT arrive before 9 a.m. (\(650 - 585\)) gives 65. Subtracting 90 campers instead of 90% (\(650 - 90\)) gives 560. Reading 90% as 90 campers gives 90.) That is 585 campers.