IMO 1969 LL FRA23

Origin: FRA

Problem

Consider the integer d = ab−1 c , where a, b, and c are positive integers and c \leqa. Prove that the set G of integers that are between 1 and d and relatively prime to d (the number of such integers is denoted by ϕ(d)) can be partitioned into n subsets, each of which consists of b elements. What can be said about the rational number ϕ(d) b ?