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jmc

prealgebra intermediate

Problem

If an integer ends in the digit and the sum of its digits is divisible by , then how many of the numbers necessarily divide it?
Solution
Since the integer ends in , it is divisible by and . Because the sum of its digits is divisible by , the number is divisible by , and we know that a number divisible by both and is also divisible by . If the number is , however, then it is not divisible by , , or . So exactly of the numbers in the original problem statement must divide it.
Final answer
4