Percentage Mental Math Shortcuts

The Core Trick: Start with 10%

To find 10% of any number, just move the decimal point one place to the left.
10% of 80 = 8 | 10% of 250 = 25 | 10% of 7 = 0.7

Once you know 10%, you can build almost any common percentage from it.

Building Other Percentages

5% → Half of 10%

Find 10%, then divide by 2.

5% of 60: 10% = 6, so 5% = 3

20% → Double 10%

Find 10%, then multiply by 2.

20% of 150: 10% = 15, so 20% = 30

25% → Divide by 4

25% is one quarter. Divide the number by 4.

25% of 80: 80 ÷ 4 = 20

50% → Divide by 2

50% is one half. Just divide by 2.

50% of 90: 90 ÷ 2 = 45

15% → 10% + 5%

Find 10%, find half of it (5%), and add them together.

15% of 80: 10% = 8, 5% = 4, so 15% = 12

The Flip Trick

8% of 50 = 50% of 8.
You can swap the percent and the number — the answer is the same. Always pick the easier calculation.

Why? Because multiplication is commutative: 0.08 × 50 = 0.50 × 8 = 4.

This works for any combination. 4% of 75 = 75% of 4 = 3. Pick whichever is easier to do in your head.

Quick Reference

To find…	Do this	Example (of 200)
10%	Move decimal left	20
5%	Half of 10%	10
20%	Double 10%	40
25%	Divide by 4	50
50%	Divide by 2	100
1%	Move decimal left twice	2

🎯 Practice Problems

1. What is 10% of 360?

A) 3.6

B) 36

C) 360

D) 0.36

2. What is 20% of 45?

A) 4.5

B) 9

C) 20

D) 22.5

3. What is 50% of 130?

A) 50

B) 65

C) 13

D) 26

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🎯 Bonus Practice

1. What is 25% of 48?

A) 24

B) 16

C) 12

D) 6

2. Using the flip trick, what is 4% of 50?

A) 4

B) 50

C) 2

D) 20

3. What is 15% of 60?

A) 6

B) 15

C) 9

D) 12

4. What is 5% of 240?

A) 24

B) 12

C) 48

D) 5

5. A meal costs $80. You want to leave a 20% tip. How much is the tip?

A) $8

B) $20

C) $16

D) $12