Mental math is not about replacing calculators — it is about building intuition so you catch mistakes and make faster decisions in stores, restaurants, and classrooms.
The 10% anchor
To find 10% of any number, move the decimal one place left. From there:
- Double it for 20%
- Halve the 10% for 5%
- Add 10% + 5% for 15%
Example
10% of $84 is $8.40. A 15% tip is $8.40 + $4.20 = $12.60.
Round strategically
Round one number up and another down to cancel error. Estimating 19 × 21 as 20 × 20 = 400 is often close enough for shopping decisions.