Square numbers ending in 5
65² in two seconds: multiply the front by the next number up, then write 25.
- Take a number ending in 5, like 65.
- Take the digits in front of the 5 (6) and multiply by the next number up: 6 × 7 = 42.
- Write 25 after it: 4225.
Try it now: Square 35, 85 and 105. (1225, 7225, 11025.)
Next crumbs
- 🧮 Mental mathMultiply any two-digit number by 11Split the digits apart and put their sum in the middle.
- 🧮 Mental mathSwap a percentage to make it easy8% of 25 is hard. 25% of 8 is easy, and it's the same number.
- 🧮 Mental mathWork out a tip in your headMove the decimal point once, then double it or add half.