OlympiadEasyNumber Theory5–6

Digit Sum

Soma dos Dígitos

A number is divisible by 9 if and only if the sum of its digits is divisible by 9. Is 123456789 divisible by 9?

You never have to divide the nine-digit number itself. Pair the digits from the two ends — $1+9$ , $2+8$ , and so on — and the sum falls out in your head.

Solution

Step 1 of 3

1.Sum of digits $= 1+2+3+4+5+6+7+8+9 = 45$ .