Let's say I have the following balls:
RRRWW
(R: red, W: white)
Then there are 5 choose 3 combinations, since:
5 * 4 * 3 different ways to arrange the red balls and then divide by 3! since the red balls are indistinguishable. Then automatically you take care of the white balls.
But what if you have:
RRRWWGG
How many different ways can you arrange this now? (G: green)