I do kinda of, but for someone reason I doubt I'm getting the answer.

A cashier has $645 in 10 dollar and 5 dollar bills. There are 90 bills in all. How many of each does the cashier have?

10x + 5x = 645

15x=645

645-90=555

I know , I know this. but for some reason I can'tfigure it out. lol

This is just 2 point for extra credit. got the other 14 questions right. Only had to do 8 of 15.

Thanks