Percentages · level 1
Percentages from 10% and 1%
Every percentage is built from two blocks you can read off instantly: 10% is the number with the decimal point moved one place left, 1% two places. Count how many of each you need and add.
Worked example: 35% of 240
10%24
1%2.4
35%84
Step by step
- 1
Find 10% — move the decimal point one place left
10% of 240 = 24
- 2
Find 1% — move it two places left
1% of 240 = 2.4
- 3
Build 35% out of those blocks
3 × 24 + 5 × 2.4 = 84
35% = 3 lots of 10% plus 5 lots of 1%.
35% of 240 = 84
The theory behind it
Intuition
Any percentage is built from 10% and 1% blocks: 17% = 10% + 5% + 2%. Because x% of y equals y% of x, you can always take the easier direction.
Common pitfalls
- ×Adding percentage points to percentage changes — a rise from 4% to 6% margin is +2pp, not +2%.
- ×Reversing a discount by adding the same percentage back: −20% then +20% does not return to par.
In the interview
Margin, growth, share and discount questions — the single most common arithmetic in an interview.