Mathematics - Math tricks

9 * x = 10 * x - x

Instead of subtracting number, it's sometimes easier to think how much you need to add to the smaller number to get to the bigger number. For example 1134 -1123. You need to add 11 to 1123 to get to 1134.
You can multiply any large number by 9 easily in your head. Example 123 * 9. Instead of calculating the number using 9, you can instead do 123 * 10 = 1230 and then subtract 123. Which gives 1107. The same trick works with 11 but you add instead of subtracting the number.
Since doubling a number is pretty easy, you can multiply any number by any power of 2 by repeatedly doubling the number.
You can square any number that ends in 5 easily. The result of every square that ends in 5 will end in 25. To get the preceding digits of 25 you simply need to multiply the digits to the left of the 5 with 1 more than that value. Example 95*95. First write down 25. Then before that 25 write down 9*10 = 90. So the result is 9025. Example 2: 1365 * 1365 = 1863225