You are doing nothing wrong except the calculus itself
Find the sum accurate to 1 decimal place, of the 20 terms of the geometric series having T1=15, and T2=20 answer is 14145.2
I do r= 20/15
so 15((20/15)^20-1) over (20/15)-1
and I get 14187?
I dont totally understand the question, what is it Im doing wrong?