FEBRUARY 13, 1994

You have an aquarium with tropical fish, some of which are silver with blue stripes, while others are blue with silver stripes. One evening, you notice the cat staring at the water. Suddenly you notice that each silver fish with blue stripes has as many identically striped tankmates as oppositely striped ones, with each blue fish with silver stripes has only half as many identically striped tankmates as oppositely stripes ones. How can that be? And what is the lowest number of fish that must be in the tank?