I feel like this is fairly basic question, but I just can't wrap my head around it.
Two red, four green blocks are placed randomly in a row. Find out probability density function and cumulative distribution function for the random variable X, when X is the number of green blocks between the red blocks.
I started with thinking about the different ways the blocks could be laid out.
4 green blocks
RGGGGR - 1! * 4! * 1! = 24
3 greens blocks
RGGGRG - 1! * 3! * 1! * 1! = 6
RGGRGG - 1! * 2! * 1! * 2! = 4
RGRGGG - 1! * 1! * 1! * 3! = 6
RRGGGG - 2! * 4! = 96
But at this point I'm completely stuck. I don't think this is even heading the right way.