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Build from 10%, 5% and 1%: a mental math trick

When to use it: Finding any percent of an amount.

Worked example

15% of $80, built from easy piecesA bar for $80 cut into tenths. 10% is $8; 5% is $4. Together, 15% is $12.10% is $85% is $415% is $12$80 in all
  1. 10% is the amount divided by 10.
  2. 5% is half of 10%.
  3. 1% is the amount divided by 100.
  4. Add the pieces you need: 15% = 10% + 5%.

example 15% of $80

  1. 10% of $80 is $8.
  2. 5% is half of that: $4.
  3. $8 + $4 = $12.

Answer: $12

Try three

  1. 35% of $540

    Show the answer

    $189

  2. 15% of $160

    Show the answer

    $24

  3. 1% of $300

    Show the answer

    $3

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Used in: Percents · First Business Story · Percentages in Business

More mental math tricks

This technique and its worked example are Doing Math’s own lesson text, checked by a blind re-solve and a review on September 27, 2026.