After 10 weeks, he'll deposit a total of 15*10 = 150 dollars on top of the $70 already in the account. This means the grand total is 150+70 = 220 dollars
If you need to use algebra, then an algebraic expression you can use is 70+15x where x is the number of weeks. Replace x with 10 and you'll end up with 70+15*10 = 70+150 = 220. It's effectively the same idea and steps as the previous paragraph, but now we're involving a variable.
