1. The problem statement, all variables and given/known data Grand unification theories predict that the proton has a long but finite lifetime. Current experiments based on detecting the decay of protons in water infer that this lifetime is at least 10^32 years. Assume 10^32 years is, in fact, the mean lifetime of the proton. Estimate the expected time between proton decays that occur in the water of a filled Olympic-size swimming pool. An Olympic-size swimming pool is 100m x 25m x 2.0m. 2. Relevant equations not sure 3. The attempt at a solution I am not really sure what the question is asking. Clearly the volume of the pool matters, otherwise it probably wouldn't be given, but I don't know where to start. Can anybody offer an explanation as to where I should start. Any help would be much appreciated.