The problem discussed in this paper is a rather specialised problem associated with a particular random walk with reflecting barriers. Such processes are discussed in particular by Feller (2). It happens that probabilities at any time associated with the system here discussed are relatively easy to determine and it is this fact which makes the given solution possible. This problem arose in certain investigations concerned with computing machines carried out by Mr. A. E. Roy of the Astronomy Department in this University and I wish to thank Mr. Roy for bringing it to my notice.