6 weeks
since he already has 20 you can subtract that from the total
110 - 20 = 90
so now 15/wk, you need 90
90/ 15 = 6
so in 6 weeks he will get 90 dollars plus the 20 he already has previously!
90+ 20= 110!
so 6 weeks!
i would really appreciate brainliest if you can thank you