This celphone plan has a fixed cost of $15, this means that regarding if you use it or not, you'll pay at least $15.
Also, added to those fixed costs, you pay $0.10 per minute for phone calls.
Acording to the text, the bill for last month was $32.50, first step is to remove the fixed charges to know how much was spent on phone calls:
$32.50-$15= $17.5
Now using cross multiplication you can calculate how many minutes she talked to spend $17.5:
$0.10____1min
$17.50___xmin
She talked 175 minutes over the phone