Quick Multiplication up to 20 x 20

“I’m having trouble above 10×10.”

This was a statement I heard many times while interacting with students preparing for competitive examinations including CAT. This was in response to my appeal to them to memorize tables up to 20×20.

Today I am posting here on QuickerMaths.com, the method which I recommend to my students too.

How to multiply up to 20×20 in your head?

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

I am trying to explain this with an example,

Say 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, 6×3 = 18

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

Here, 190 + 18 = 208

Simple!! Isn’t it?

Another example, 17×19 =  (17+9)*10 + (7*9)  = 260 + 63 = 323

(10+a) * (10+b) = 100 + 10b + 10a + a*b
= (10+a+b) * 10 + a*b
==========
This can be extended to sums like 23 * 28
(20+a) * (20+b) = 400 + 20b + 20a + a*b
= (20+a+b) * 20 + a*b
So, 23*28 = (23+8)*20 + 3*8 = 620 + 24 = 644.
==========
& so on.

by NANDEESH H N

Try for yourself. Let me know (by posting a comment below) if you liked it.

Click here for some more fast multiplication tricks

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