In how many ways can a committee of 12 people, of which 5 must be in the 21−30 age group, 4 must be in the 31−40 age group and 3 must be in the 41−50 age group, be chosen if there are 6 people of each age group?

Answer 1

how many possibilities in each subgroup? ... 6Cn

how many ways to combine the different subgroups?

Answer 2

1800. Notice the words be chosen, those are usually in the context of multiplication. Then it's permutations because it's people. (6 permutation 5) * (6 permutation 4) * (6 permutation 3)