You cannot tell the marbles of the same color apart, so we have
21 positions
o o o o o o o o o o o o o o o o o o o o o
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
That's 21 choose 10 positions to place the 10 red marbles in. That's C(21,10)
That's leaves 11 choose 5 positions to place the white marbles in. That's
C(11,5). That leaves 6 choose 6 positions to place the blue marbles in.
Answer C(21,10)·C(11,5)·C(6,6) = 352716·462·1 = 162954792
Edwin
