Please use sigma notation

Exercise 1. For this problem assume all movements are performed in a straight line, and assume we want to travel a certain distance Xkm. We define the first Zeno step as the point where we have travelled nine tenths of the total distance then we have finished our first step. We define the second Zeno step as the point where we have travelled nine tenths of the remaining distance after the first Zeno step. And in general, after having travelled nine tenths of the remaining distance on the N-th Zeno step we consider that we have completed the N+1-th Zeno step. Alice is a great swimmer and is able to swim at a constant speed of 20 km/h, and today she wishes to swim 10km. For each N, determine the time Alice will need to perform the N-th Zeno step