A natural number is divisible by 3 if and only if the sum of its digits is divisible by 3.

The sum of the digits of the number 717 is 15 (7 + 1 + 7 = 15). Note that the sum of the digits of the number 15 is 6 (1 + 5 = 6).

717 is divisible by 3 since the sum of the digits of 717 is divisible by 3 (15 ÷ 3 = 5). Note also that the sum of the digits of 15 is divisible by 3 (6 ÷ 3 = 2).

The sum of the digits of the number 716 is 14 (7 + 1 + 6 = 14). Note that the sum of the digits of the number 14 is 5 (1 + 4 = 5).

716 is NOT divisible by 3 since the sum of the digits of 716 is NOT divisible by 3 (14 ÷ 3 = 4 r 2). Note also that the sum of the digits of 14 is NOT divisible by 3 (5 ÷ 3 = 1 r 2).

716 123 456 789 098 765 432 104 is divisible by 3 since 108 and 9 are divisible by 3.

NOTE: 7 + 1 + 6 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 0 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + 0 + 4 = 108

NOTE: 1 + 0 + 8 = 9

717 123 456 789 098 765 432 108 is NOT divisible by 3 since 113 and 5 are NOT divisible by 3.

NOTE: 7 + 1 + 7 + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 0 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + 0 + 8 = 113

NOTE: 1 + 1 + 3 = 5