Inspired by information in the Math is Cool thread, I thought it would be great to pull ideas together that can make our mathematical lives easier. The shortcut doesn’t have to be earth-shattering, just something that can save some time.

To multiply mentally by 11:

24 x 11 = 264. Add the two and four (ones and tens digits) to get the 6 (tens digit in the sum). Keep the ones digit the same. Move the 2 (tens digit) into the hundreds position in the sum.

If you get more than 9 when you total your digits, simply carry the one onto the hundreds digit. Ex. 84 x 11= 924.

To Multiply mentally by 5:

26 x 5 = 130. Take half of the number 26 and get 13. Move the decimal once to the right and get 130. Basically dividing by two then multiplying by ten.

Division by 5 works just the opposite way.

27/5 = 5.4 Double 27 and get 54. Move the decimal once to the right to get 5.4.

Come on all you math geeks and help me out.