Suppose you have $28.00 in your bank account and start saving $18.25 every week. Your friend has $161.00 in his account and is withdrawing $15 every week. When will your account balance be the same?

28 + 18.25x = 161 - 15x (here, x stands for every week this addition or withdrawal happens)

So:

28+18.25x = 161 - 15x

18.25x = 161 - 28 - 15x

18.25x = 133 - 15x

18.25x + 15x = 133

33.25x = 133

x = 133/33.25

x = 4

So, they will have the same balance in 4 weeks.