Steven wants to buy a $500 bicycle. Steven has no money saved, but will be able to deposit $45 into a savings account when he receives his paycheck each Friday. However, before Steven can buy the bike, he must give his sister $85 that he owes her. For how many weeks will Steven need to deposit money into his savings account before he can pay back his sister and buy the bike?

The answer is 13 weeks

Math is not my forte, so correct me if I am wrong.

If he first has to pay back $85 to his sister, then he'd be able to do that by the second week. 45+45 is 90 so he will have $5 once he pays back his sister. In 3 weeks, He'll have 50 dollars. In four weeks, he'll have 95 dollars. In five weeks, he'll have 140 dollars. In six weeks, he'll have 185 dollars. In several weeks, he'll have 230 dollars. In eight weeks, he'll have 275 dollars. I think you get the gist by now. I calculated that in 13 weeks he will have enough money to pay back his sister and buy his bike.