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Percentage Building Blocks: 10%, 5%, 1%

Level 1 Mental percentages · Reviewed by The NumThink editorial team · Last reviewed

Short version: Find 10% (shift the decimal), 5% (half of 10%), 1% (shift twice) — and combine: 15% of 240 = 24 + 12 = 36.

This page is part of the NumThink Mental percentages course. Read why the method works, watch it run on worked examples, then drill it in the practice box below until it feels automatic.

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Why it works

Percent means "per hundred", so 10% is literally the number ÷ 10 — one decimal shift. 5% is half of that, and 1% is a second shift. These three blocks add up to almost every percentage in daily life.

Combining blocks works because percentage is linear: 15% of 240 = (10% + 5%) of 240 = 10% of 240 + 5% of 240.

The steps

  1. Find 10%. 240 → 24.1? No: one decimal shift — 24.
  2. Find 5% (half of 10%). 24 ÷ 2 = 12.
  3. Add the blocks. 15% = 10% + 5% → 24 + 12 = 36.

Worked examples

Easy 10% of 240

Show the thinking

240 ÷ 10 = 24

That is all — one shift.

Answer: 24

Medium 15% of 240

Show the thinking

10% of 240 = 24

5% of 240 = 12

24 + 12 = 36.

Answer: 36

Hard 21% of 300

Show the thinking

10% of 300 = 30

10% again = 30 (that is 20%)

1% of 300 = 3

30 + 30 + 3 = 63.

Answer: 63

When to use it

  • Almost every mental percentage — tips (15–20%), discounts (10/25/30%), taxes.
  • Percentages like 12% (10% + 2×1%) or 35% (3×10% + 5%).

When NOT to use it

  • 25%/50%/75% — quarters are faster (see the quarters method).
  • Percentages of awkward bases where blocks create decimals — the swap trick may rescue you.

Common mistakes

Misplacing the decimal. 10% of 240 answered as 2.4 or 240. Fix: 10% = "move one place left", always. Say "twenty-four" not "two-point-four" for 240.
Adding percentages of different numbers. Computing 5% of the 10% result (12% of 240 as 24 + 1.2). Fix: Every block is taken from the ORIGINAL number, then summed.
Check yourself: A 10% block can never exceed the number itself, and 5% is always half of your 10% — quick sanity checks on every step.