Multiply by 5: halve it, then add a zero
5 is half of 10. So 48 × 5 is just half of 48 with a zero on the end: 240.
- Take the number, like 48.
- Halve it: 24.
- Multiply by 10 by adding a zero: 240.
- If it's odd, add the zero first, then halve: 37 × 5 → 370 → 185.
Try it now: Do 64 × 5, 86 × 5 and 23 × 5. [320, 430 and 115.]
Next crumbs
- 🧮 Mental mathMultiply by 25: divide by 4, then add two zerosFour quarters make a dollar. So 36 × 25 is 36 ÷ 4 = 9, then two zeros: 900.
- 🧮 Mental mathMultiply any two-digit number by 11Split the digits apart and put their sum in the middle.
- 🧮 Mental mathConvert Celsius to Fahrenheit in your headDouble it, knock off a tenth, add 32. That's the exact formula, just in friendly pieces.