Basic Concept Of Math's .

How to multiply up to 20x20 in your head?

Assumption: You know your multiplication table reasonably well up to 10×10.

If you want to multiply - 16 x 13 .

Step 1 – Add the unit’s digit of one to the other number –

Here, add 16 + 3 = 19

Or, add 13 + 6 = 19

Step 2 – Put a zero after the number (i.e. multiply it by 10)

Here, 19 becomes 190

Step 3 – Multiply unit’s digit of both the numbers

Here, 6x3 = 18

Step 4 – Add the product to the result of Step 2

Here, 190 + 18 = 208

So Simple !

Another example, 17x19 = (17+9)*10 + (7*9) = 260 + 63 = 323