learn your 11-20 times tables

[CanadaAotS]

In just FIVE minutes you should learn to quickly multiply up to 20x20 in your head. With this trick, you will be able to multiply any two numbers from 11 to 19 in your head quickly, without the use of a calculator.

I will assume that you know your multiplication table reasonably well up to 10x10.

 

Try this:

 

 

Take 15 x 13 for an example.

Always place the larger number of the two on top in your mind.

Then draw the shape of Africa mentally so it covers the 15 and the 3 from the 13 below. Those covered numbers are all you need.

First add 15 + 3 = 18

Add a zero behind it (multiply by 10) to get 180.

Multiply the covered lower 3 x the single digit above it the "5" (3x5= 15)

Add 180 + 15 = 195.

 

This is interesting actually...

 

Following the instructions:

10+x*10+y

10+x+y

100+10x+10y

100+10x+10y+xy

 

Using FOIL:

100+10x+10y+xy

 

This thread is for math tricks in general...

If anyone has a math trick thats stumping them just post it here

[the tree]

Bleh, I never learnt my times tables, never saw the point really. Newton didn't either apparently, he used log tables for everything, crazy man.

[insane_alien]

why when i have my BSC(Big Shiny Calculator)?

[abskebabs]

Sometimes I like to challenge myself with a bit of mental arithmetic in my head now and again(instead of using a calculator), especially 2 digit multiplication like that shown above. I like the look of CanadaAOT's method above, though I have been using a different method myself.

[Callipygous]

that seems like it would be more of a pain to remember each time than just being able to multiply two digit numbers in my head : P