3 Answers

7 boxes of pencils = 70 pencils
5 boxes of erasers = 70 erasers
(What’s the lowest common multiple of 10 and 14? It’s 70.)

14 boxes of pencils and 10 boxes of erasers would work (140 of each)
The least she should buy is 7 boxes of pencils and 5 boxes of erasers (70 of each)