Hi guys,

I am a little stuck on where to begin here and any help would be great.

This is the question I am faced with:

Now, the formula I have been using recently for working out the sum of a geometric series to n places is:

$\displaystyle S_{n} = \frac{a(1-r^n)}{1-r}$

But I am unsure how to use it in this context.

Thanks for any help!