Originally Posted by

**peejaygee** Firstly, I know how to find the number of terms of Un=ar^(n-1) using logs. However, I am stuck on a question using the formula; Sn=a(1-r^n)/1-r

I assumed that the process would be the same with rearanging but I am not getting correct answer.

The question is;

A post is being driven into the ground by a mechanical hammer. The distance it is driven by the first blow is 8cm. Each subsequent blow drives the post 90% of the distance of the proceeding blows. If each blow takes 10 seconds how long will it take to drive the post 90% of the distance of the proceeding blow?