Sum of Geometric Progression

Hi guys, need some help with this question:

The sum of the first 20 terms of a geometric series is 10 and the sum of the first 30 terms is 91, find the sum of the first 10 terms.

So this is how i did it:

I tried to find the common ratio:

S20 = 10

a(r^20 - 1)/(r-1) = 10

10(r-1) = a(r^20 - 1)

(r-1) = (a/10)(r^20 - 1) - EQN 1

S30 = 91

a(r^30 - 1)/(r-1) = 91

Sub. EQN 1 into the above EQN:

a(r^30 - 1) * 10/a(r^20 - 1) = 91

THe solution just becomes more and more complicated... =/ is there something wrong? How do I simplify?

Thanks in advance!