here are 7 balls. Each ball has one color out of the 7 colors of the rainbow. We have to choose 4 balls out of the 7, among these 4 balls, one must be red, and another one must be green. We then place these 4 selected balls on top of a chair . there are 4 chairs ( each chair for one ball only) . How many ways are there to choose 4 balls randomly and place on top of 4 chairs