I am having trouble dealing with this question:

A ward in a city hospital has 15 nurses due to work on Friday. There are 3 shifts that need to be staffed by 5 nurses on each shift. How many different arrangements for staffing these 3 shifts are possible, assuming that each nurse only works 1 shift?

The answer is 756756 arrangements.

But i don't understand why this is the answer though.

Can somebody please help me with this question?