INMO 1988 Problem 3

Five men, $A$ , $B$ , $C$ , $D$ , $E$ are wearing caps of black or white colour without each knowing the colour of his cap. It is known that a man wearing black cap always speaks the truth while the ones wearing white always tell lies. If they make the following statements, find the colour worn by each of them: $A$ : I see three black caps and one white cap. $B$ : I see four white caps $C$ : I see one black cap and three white caps $D$ : I see your four black caps.