by 2: last digit even (0,2,4,6,8)
by 3: sum of digits divisible by 3
by 4: last two digits divisible by 4
by 5: last digit 0 or 5
by 6: divisible by 2 and 3
by 7: double last digit, subtract from rest; repeat
by 8: last three digits divisible by 8
by 9: sum of digits divisible by 9
by 10: last digit 0
by 11: alternating sum of digits divisible by 11
by 12: divisible by 3 and 4
by 13: multiply last digit by 4, add to rest; repeat