Where are you in your math? What level?
The only way you arrive is if you are allowed to take an infinite amount of steps. Otherwise you will not reach the wall one by adding 1/2, 1/4, 1/8 etc. If you are able to write sums you basically should be able to show that for n from 1 to infinity the sum over 1 devided by n equals one. You can even write a little computer program to see how quickly it converges.
I looked for some info online and this one is excellent:
http://members.aol.com/kiekeben/zeno.html
George