Question: numbers are divisible by 9?
Answer: A number is divisible by 9, if the sum is a multiple of 9 or if the sum of its digits is divisible by 9. Consider the following numbers which are divisible by 9, using the test of divisibility by 9: 99, 198, 171, 9990, 3411. Sum of the digits of 99 = 9 + 9 = 18, which is divisible by 9.