Q92) Consider the set G of all integers between 100 and 9999 (including the extremes). Call two integers a and b in G to be in the same category if the digits appearing in a and b are the same. For example, if a = 101, b = 100, c = 1000 and d = 120, then a, b and c are in the same category but c and d are not. Find the number of different categories that can be formed out of G.