Multiply by 25: divide by 4, then add two zeros
Four quarters make a dollar. So 36 × 25 is 36 ÷ 4 = 9, then two zeros: 900.
- Take the number, like 36.
- Divide it by 4: 9 (halve it twice if that's easier).
- Multiply by 100 by adding two zeros: 900.
- If it doesn't divide evenly, keep the decimal: 18 ÷ 4 = 4.5, so 18 × 25 = 450.
Tip: The same idea works for 50 (halve, add two zeros) and 125 (divide by 8, add three zeros).
Try it now: Do 44 × 25, 28 × 25 and 14 × 25. [1100, 700 and 350.]
Next crumbs
- 🧮 Mental mathMultiply by 5: halve it, then add a zero5 is half of 10. So 48 × 5 is just half of 48 with a zero on the end: 240.
- 🧮 Mental mathSwap a percentage to make it easy8% of 25 is hard. 25% of 8 is easy, and it's the same number.
- 🧮 Mental mathConvert Celsius to Fahrenheit in your headDouble it, knock off a tenth, add 32. That's the exact formula, just in friendly pieces.