[mathematics] calculation tricks

[str33tl0rd]

hello all,
i thought of making a thread where everybody would mention a few simple maths calculation tricks....i will start with my first one:

Multiplying 2 digit numbers with a base of 10

e.g: 13 x 12
1. add the first number to the second digit of the 2nd number, so it becomes in this case:
13+2=15 15 now that is our first two digits....

2. multiply the second digit of the first number and the second by each other:
2 x 3=6 6 this becomes the last digit

so our answer is 156

but what if the numbers multiplied gave another two digit number, for example: 13 x 15, 3x 5= 15 here simply put the one over the other digit and add to it just like normal addition so it will be:
1. 13+5=18; 3x5=15, 1815 the one is carried over to 8, to make it 195. and thats your answer....=]