If $\displaystyle -1<r<1$ and $\displaystyle S_n=\displaystyle \sum^n_{p=1} r^p$ find

a) $\displaystyle \displaystyle \lim_{n\rightarrow\infty} S_n$

b) $\displaystyle \displaystyle \sum^{\infty}_{n=1}nr^n$

c) $\displaystyle \displaystyle \sum^n_{q=1} S_q$

I have done the first two correctly, but I have no idea what the third one means. This is the entire question.

Thanks for any help.