Question 206084
To check if a number is divisible by 45, you are correct: you check to see if it is divisible by both 5 and 9. Since the last digit of 31120110 is a 0, this means that it is divisible by 5. Because the digits add up to 9 (3+1+1+2+0+1+1+0=9), this means that the number is also divisible by 9. So this means that 31120110 is indeed divisible by 45. 


Note: as a check, {{{31120110/9=691558}}} which confirms our answer.



Note: this theory works for any composite number. For instance, the divisibility rule for 6 states that any number is divisible by 6 if it is both divisible by 2 and 3 (which multiply to 6).