divisible by 7
Tuesday, 16th October, 2007 at 2:25 pm
I really like this kind of mathematical tricks…
Do you need to know if a number is divisble by 7?
Easy! You have to multiply the last figure from your number by 2 and then, rest the result to the original number less this last figure. If the result is divisible by 7, or 0, then the original number is also divisible by 7.
Example:
133
3 x 2 = 6
13 – 6 = 7
7 is divisble by 7 (1), 133 is then divisible by 7
4578
8 x 2 = 16
457 – 16 = 441
now same with 441 (recursive)
1 x 2 = 2
44 – 2 = 42
42 is divisble by 7 (6), 4578 is then divisible by 7
If you don’t know the 7 times table 
you can continue the function until find number 7 or 0.
42
2 x 2 = 4
4 – 4 = 0
via microsiervos, espejo-ludico
