Hello, I need help with the attached question. I understand random walks but don't quite get setting up this problem

1. (Old friends) Alice and Bob are very old friends who haven't talked to each other for a long time. Now, after all these years, they happen to live on the same innitely long street. One sunny day both Alice and Bob decide to go shopping. They start at integer positions a % and 115 % and pb %. (d) Having done all of this, can you nd a sharper (more inclusive than the previous one) relationship between pa and 1);, that will yield a nite expectation