Because like terms you add or subtract together to make it easier to do the problem. just like you do with whole numbers. you add an subtract to make the answer simpler.
if she buys 10 of the small juice box packages and six of the yogurt treat packages then she should have an even number of both. That would be 60 of each.
18 - [ 18 - [ 18 - ( 18 - 1)]]
= 18 - [ 18 - [ 18 - 17]]
=18 - [ 18 - 1]
= 18 -17
=1
