Pencils are sold in boxes of 10, erasers are sold in boxes of 14, a teacher wants to buy the same number of pencils and erasers. work out the smallest number of boxes of each item she should buy. what' s the answer? ? i don't get it

10 = 2 x 5 
14 = 2 x 7 

In order to *cover* those two numbers, you need a 2, a 5 and a 7. 
2 x 5 x 7 
= 70 

That's the lowest common multiple. 

Now just divide to get the number of boxes. 
70 / 10 = 7 boxes of pencils 
70 / 14 = 5 boxes of erasers