No one is answering because your problem makes no sense.
Please review the wording of the last sentence.
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?
Obviously, the passage in blue is incorrect.
Please supply the correct distance.