for how many integers a,between 30 and 40 is it true that 5/a,8/a and 13/a are all in lowest terms

题目

for how many integers a,between 30 and 40 is it true that 5/a,8/a and 13/a are all in lowest terms
please help...i don't understand the question.

答案

就是说 在30 到 40 之间分别有多少数能被5 8 13 整除
答案是分别有 3个 2个 1个