Square 85 instantly

Question

Solve in your head. What is 85^2? Aim: under 3 seconds.

Accepted Answer

For any number ending in 5, write it as 10n + 5. Then (10n+5)^2 = 100\,n(n+1) + 25. So: take the leading digit(s) n, multiply by n+1, and append 25. For 85: n = 8, so 8 	imes 9 = 72, append 25: 85^2 = 7225. Takeaway. "Multiply the front by one more than itself, stick 25 on the end." This works for 115^2 (11 	imes 12 = 132 	o 13225), 135^2 (13 	imes 14 = 182 	o 18225), and so on. It should be automatic before any mental-math screen.

Square 85 instantly

Your answer

More Mental Math questions