geometricsequences

Can someone please explain how the worked solution got (7/8)^n-1 for part (ii)? If we are finding the geometric sum, wouldn't r = 7/8?

I'm finishing off holiday work assigned to me before school starts back and I find myself stuck on this question all the time: "In any GP, let Rn = T(n + 1) + T(n + 2) + ... + T(2n). Show that Rn:Sn = r^n:1, and hence find r if R8:S8 = 1:81" This is how the question is printed in my textbook...