What Are Mental Percentages?
Calculating percentages mentally means finding a fraction of a number without a calculator. The most useful percentages — 10%, 25%, 50%, and their combinations — cover the vast majority of real-world situations: tips, sales tax, discounts, and interest rates.
Why Learn Mental Percentages?
Every shopping trip, restaurant visit, and financial decision involves percentages. Being able to quickly compute a 15% tip or a 30% discount saves time and prevents overpaying. This skill also builds intuition for data interpretation in news, statistics, and business.
Key Percentage Shortcuts
10%: Move the decimal point one place left. 10% of 85 = 8.5. This is your master key — most other percentages build from 10%.
50%: Halve the number. 50% of 120 = 60.
25%: Divide by 4 (or halve twice). 25% of 120 = 30.
5%: Find 10%, then halve it. 5% of 64 = 3.20.
15%: Add 10% and 5%. 15% of 64 = 6.40 + 3.20 = 9.60.
20%: Find 10%, then double it. 20% of 50 = 5 + 5 = 10.
Combining Percentages
For unusual percentages, decompose into known parts. Need 35%? Calculate 25% + 10%. Need 17%? Calculate 10% + 5% + 2% (where 2% is one-fifth of 10%). With practice, these decompositions become instant.
Practice Examples
Example 1: What is 10% of 85?
- Move decimal one place left
- 85 → 8.5
Answer: 8.50
Example 2: What is 25% of 120?
- 25% = one quarter
- 120 ÷ 4 = 30
Answer: 30
Example 3: What is 15% tip on $64?
- 10% of 64 = 6.40
- 5% of 64 = 3.20 (half of 10%)
- 6.40 + 3.20 = 9.60
Answer: $9.60