How many ways can 21 students be sorted into 3 groups of 7 s

How many ways can 21 students be sorted into 3 groups of 7 students each?

Solution

(21C7 x 14C7 X 7C7 ) / 3!

1st we are selecting 7 students out of 21. then left is 14. then selecting 7 students out of 14. then left are 7 students. selecting 7 out of 7.

3! is for the number of groups since the groups are identical.