Who mad the atomic bomb?

Answer 1

1. All round numbers are divisible by 1
2. If it's an even number (i.e. the last digit is a 0, 2, 4, 6 or 8), then it's divisible by 2.
3. If the sum of digits is divisible by three, then the number is divisible by three (e.g. the number 2821446 is divisible by three, because 2 + 8 + 2 + 1 + 4 + 4 + 6 equals 27, which is also divisible by three. Not sure of that one? 2 + 7 = 9, and 9 is indeed divisible by three).
4. If the last two digits are divisible by 4, then the number itself is.
5. If the last digit is a 0 or a 5, then it's divisible by 5.
6. If it's and even number, and the sum of the digits is divisible by three, then it is divisible by six.
7. Take the last digit, double it, and subtract it from the remaining digits. If the result is divisible by seven, then the original number is as well. Take the number 2422 for example. If we double the last digit and subtract it from the rest, we get 242 - (2 * 2) = 242 - 4 = 238. Not sure? Let's try that again on our result: 23 - 2 * 8 = 26 - 16 = 7. 7 is indeed divisible by 7, so we no that 242 is and in turn, 2422 is as as well.
8. If the last three digits are divisible by eight, then the whole number is as well. Usually it's easiest to take the last three digits, and try dividing that by 2 three times in a row. If the result is a whole number, then the original is divisible by eight.
9. If the sum of the digits is divisible by 9, then the number itself is. For example let's sum up the digits of 83592: 8 + 3 + 5 + 9 + 2 = 27. We can again repeat that: 2 + 7 = 9. 9 is obviously a multiple of 9, so we know that our our original number is as well.
10. Any round number that ends with a zero is divisible by 10.