This works for
whole numbers. Simply
add up the digits in the number. The
resulting number should be small enough to
divide by
three in your head. If that number is evenly
divisible by three, so is the original number. I heard this long ago in
fourth grade.
Examples :
45
4 + 5 = 9
9 / 3 = 3
2550
2 + 5 + 5 + 0 = 12
12 / 3 = 4
1290363
1 + 2 + 9 + 0 + 3 + 6 + 3 = 24
24 / 3 = 8
or, if you don't like dividing 24 by 3,
24
2 + 4 = 6
6 / 3 = 2
Voila!