Originally Posted by arlen1
AFAIK, it is 'number of ways to place 4 identical balls into 3 containers' = 'throw 2 sticks between 4 balls, on the outside allowed' = C(6,2) = (6*5)/(2*1) = 15.

(3 containers, and not 4, because the number < 1000 and so has <= 3 digits.)
Ah of course. For some reason I misread it as a 4 digit number.