Thus, putting the case mathematically, Mr. Copperdock was the highest common factor of the terms representing the four dead men. Inspector Whyland had spent much time and care in investigating their histories and their actions, and he could find no other factor common to them all. They had been, so far as he had been able to ascertain, complete strangers to one another, and they could never consciously have met one another. The only characteristics which they had in common were that they had all received counters numbered in the order of their deaths, that they were all males, and that their ages had all been over fifty.