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jmc

number theory intermediate

Problem

How many positive three-digit integers with a in the units place are divisible by ?
Solution
Odd multiples of 5 have a 5 in the units place, while even multiples of 5 have a 0 in the units place. Multiples of 15 are multiples of 5, so we look at how many three-digit multiples of 15 are odd to find how many have a 5 in the units place. The three-digit multiples of 15 range from 105 to 990, or to . So there are three-digit multiples of 15. Half of them will be odd, so there are 30 odd, three-digit multiples of 15. There are positive, three-digit integers with a 5 in the units place that are divisible by 15.
Final answer
30