Whatever color is in this slot, we're going to take it out of the possible colors. Now that we've taken that color out, how many possibilities are when we go to this slot, when we go to the next slot? How many possibilities when we go to the next slot right here? We took one of the six out for the first slot, so there's only five possibilities here. By the same logic, when we go to the third slot, we've used up two of the colors already. There will only be four possible colors left. Then for the last slot, we would have used up three of the colors, so there's only three possibilities left.