L6 · Percent of a number (within 100 percent)

Learning goal

“I can find a percent of a number by setting up a proportion out of 100 - like 25/100 = n/80 - and solving it by scaling, and I can check my answer with 10% chunks on a hundred grid.”

Percent — a ratio that compares a number to 100; 25% means 25 out of 100, written 25/100.

Hundred grid — a 10 x 10 square of 100 small squares used to model percents.

Benchmark percent — an easy percent to build from: 10%, 25%, 50%, 75%.

Worked example

Find 20% of 50. Step 1: Percent means 'out of 100.' Write the percent as a ratio: 20/100. Step 2: Set up the proportion - percent ratio equals part over whole: 20/100 = n/50. Step 3: Scale: 100 -> 50 is / 2, so 20 / 2 = 10. n = 10. 20% of 50 is 10. Hundred grid check: 50 is half of 100, and 10 is half of 20 - the shaded part of the grid matches the proportion. On the double number line, 0 -> 100 (percent) aligns with 0 -> 50 (the number); the mark at 20% sits above 10. Every 'percent of a number' question is a proportion with one denominator already known - 100.

Check yourself — explain it

Percent means...

Paired problem — example still visible

Find 50% of 60 -> 30

Faded completion — last step is yours

"10% of 90: 10/100 = n/90, and 100 -> 90 is / 10 x 9, so n = ___" -> 9

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