How many ways are there to put 4 balls into 3 boxes, given that the balls are not distinguished but the boxes are?
Note that the boxes are distinguishable....so....(4, 0, 0) is different from (0, 4 , 0)
The total number of ways =
C ( n + k - 1, k) where n = the number of boxes and k = the number of balls
So we have
C ( 3 + 4 - 1 , 4) = C (6 , 4) = 15 ways
We can confirm this by noting that
(4, 0, 0) has C(3,1) = 3 arrangements
(3,1,0) has 3! = 6 arrangements
(2,2, 0) has C (3,1) = 3 arrangements
(2, 2 , 1) has C(3,1) = 3 arrangements
So
3 + 6 + 3 + 3 = 15 arrangements (ways)