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

a= 15

n=20

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?