*Square any 2 digit number ending in 5.*

For example, letâ€™s square 25.

Step 1: Take the first digit and multiply it by itself plus 1. So 2 x 3 = 6.

Step 2: Take the product from Step 1 and concatenate it with 25 on the right to get 625.

Letâ€™s try again. Square 65.

Step 1: Â 6 x (6+1) = 42.

Step 2: Append 25 to the right of 42 and you get 4225.

*Multiply any two digit number by 11.*

For example, letâ€™s multiply 78 x 11.

Step 1: Take the number being multiplied by 11 and add the digits together. Â So 7 + 8 = 15.

Step 2: Insert a zero between the same two digits. Â 708.

Step 3: Add the result from Step 1 to the zero of Step 2. In this case we have 15, so we need to carry the 1. An easy way to think about it is 70 + 15 = 85 with the 8 from 708 added at the end for 858.

Tags: Technique

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