Multiply 83 by 87 instantly

Question

Solve in your head. What is 83 	imes 87? Aim: under 5 seconds.

Accepted Answer

This is the same trick behind squaring numbers ending in 5, generalized. When two numbers share the same tens digit and their units digits add to 10 (here both start with 8, and 3 + 7 = 10), the product is: - front: the shared tens digit t times t+1, - back: the product of the two units digits, written as two digits. For 83 	imes 87: tens digit 8, so 8 	imes 9 = 72 for the front; units 3 	imes 7 = 21 for the back: 83 	imes 87 = 7221. Squaring a number ending in 5 is just the special case where both units digits are 5: the front is n(n+1) and the back is 5 	imes 5 = 25. Takeaway. "Same te…

Multiply 83 by 87 instantly

Your answer

More Mental Math questions