How many ways can 20 students be sorted into 4 groups of 5 s

How many ways can 20 students be sorted into 4 groups of 5 students each?

Combination: Picking a team of 5 people from a group of 20. C(20,5) = 20!/(15! * 5!) = 15504

Combination: Picking a team of 5 people from a group remain of 15. C(15,5) = 15!/(10! * 5!) = 3003

Combination: Picking a team of 5 people from a group of 10. C(10,5) = 10!/(5! * 5!) = 252

Combination: Picking a team of 5 people from a group of 5. C(5,5) = 1

total selection is :15504+3003+252+1=18760