Percentage Tricks
Finding 20% of a Number – Find 10%, Then Double
Twenty percent is just two lots of 10%, and you already know how to find 10%.
This page explains the rule, shows why it works, and walks through worked examples. Then you can try it yourself and check your understanding with practice questions. Use the previous and next links at the bottom to move through the tricks in order.
The rule
The Rule: To find 20% of a number, find 10% first and then double it.
Why it works
Why It Works: 20% is simply double 10%, and 10% happens to be the easiest percentage of all to find. So instead of tackling 20% head-on, this trick quietly turns it into two small, easy steps: find the easy one first, then double it.
Formal Proof (for advanced learners)
20% = 20÷100 = 2×(10÷100) = 2×10%. So computing 20% of n is algebraically identical to computing 10% of n and then multiplying that result by 2.
Examples
- 20% of 150 → 10% is 15, doubled is 30.
- 20% of 90 → 10% is 9, doubled is 18.
- 20% of 250 → 10% is 25, doubled is 50.
Step by step
Example A: Find 20% of 70
- Find 10% first: 70 ÷ 10 = 7.
- Double it: 7 × 2 = 14.
Example B: Find 20% of 95
- Find 10% first: 95 ÷ 10 = 9.5.
- Double it: 9.5 × 2 = 19.
Try it yourself
Now it is your turn. A new random question appears each time. Type your answer in the box and press Check Answer. If you are right, we show how the trick got you there, step by step. If you are not, you will see the full working two ways, the shortcut and the usual method, so you can spot exactly where your answer differed. Press New Question as often as you like: a few quick rounds is the best way to make the trick feel automatic.
Practice questions
Have a go on paper first, then tap Show answer to check your method.
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Find 20% of 30.
Show answer
6. Find 10% first: 30 ÷ 10 = 3. Double it: 3 × 2 = 6.
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Find 20% of 80.
Show answer
16. Find 10% first: 80 ÷ 10 = 8. Double it: 8 × 2 = 16.
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Find 20% of 150.
Show answer
30. Find 10% first: 150 ÷ 10 = 15. Double it: 15 × 2 = 30.
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Find 20% of 45.
Show answer
9. Find 10% first: 45 ÷ 10 = 4.5. Double it: 4.5 × 2 = 9.
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A jacket costs 260 coins and has 20% off. How many coins is the discount?
Show answer
52. Find 10% first: 260 ÷ 10 = 26. Double it: 26 × 2 = 52.
Tips and watch-outs
- 20% is the same as one fifth, so it is also the number divided by 5.
- Decimals are fine: 20% of 35 → 10% is 3.5, doubled is 7.