Try to solve in your head 65 x 65. It´s kind of difficult, right?

To make it easier, try to multiply the number in the tens (6) by itself plus 1. Then, add “25” (the square of the units) at the right of the result. So:

First step: 6 x (6 + 1) = 6 x 7 = 42
Second step: Add 25 to the right of the result

Result: 4,225. Four thousand two hundred and twenty five.

As you can see, to quickly solve in your mind a complicated operation such as the square of 65, the only thing you have to do is multiply 6 x 7, a result you have probably known since you were 7 years old.

With this same procedure, you can calculate the square of any two-digit number that ends in 5 as quickly as the example (maybe in two and a half seconds). And, with a little variation that I´ll teach you later, you will be able to do the same with numbers that end in 3 or multiples of 3.

This book describes these types of techniques, and also explains why is it important to know and use them.

Have a good reading!