Post by: Roflmao on March 12, 2009, 11:03:14 pm
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.
That is It! Wasn't that easy? Practice it on paper first!
Post by: TrueTears on March 12, 2009, 11:11:31 pm
Post by: Edmund on March 12, 2009, 11:34:27 pm
Post by: Gloamglozer on March 13, 2009, 06:32:08 pm
Post by: wombifat on March 13, 2009, 08:28:47 pm
Post by: hard on March 13, 2009, 08:39:08 pm
what? i did not understand that at allconcur
Post by: ninwa on March 13, 2009, 08:39:59 pm
what? i did not understand that at all
plz excuse my shitty Paint handwriting :P
(http://img23.imageshack.us/img23/5993/maths.png)
Post by: IntoTheNewWorld on March 13, 2009, 08:43:26 pm
Post by: hard on March 13, 2009, 08:58:07 pm
so you get 19 + 8 = 27
=
270
8+9=17
270+17=287
but the real answer is 342
Post by: AppleThief on March 13, 2009, 09:00:46 pm
hrmm but let's say 18 * 19supposed to be 8 x 9 not 8 + 9
so you get 19 + 8 = 27
=
270
8+9=17
270+17=287
but the real answer is 342
Post by: hard on March 13, 2009, 09:04:24 pm
the system workkkkkkkkkkkkks wooothrmm but let's say 18 * 19supposed to be 8 x 9 not 8 + 9
so you get 19 + 8 = 27
=
270
8+9=17
270+17=287
but the real answer is 342
Post by: Mao on March 13, 2009, 11:25:19 pm