Dorrie is having the basement remodeled. the contractor charges $17.50 per hour, plus the cost of supplies. the contractor goes to two different stores for supplies. at the first store the contractor spends $150.00 and at the second store the bill is $220.00. dorrie gets a bill at the end of the project for $720.00. she thought the bill was going to be $570.00. how many hours did the contractor work? what error did dorrie make?