To repeat, we need this:
15+c=17.50.
let's imagine a person who wants to save up for a book that costs 17.50. This person has already saved 15 dollars. how much more does he or she still need to save?
So now we solve:
15+c=17.50
subtract 15 from both sides
15+c=17.50
15-15+c=17.50-15
c=2.5
so the person needs to save 2.5 dollars more.