A basket contains 6blue, 2 red, 4 green and 3 yellow balls If 4 balls are picked at random, what is the probability that 2 are red and 2 are green?

Given that basket contains 15 balls, in which 2 red and 4 green

The probability of getting 2 red and 2 green balls is 

$\begin{array}{rcl}p& =& \frac{C\left(2,2\right)\times C\left(4,2\right)}{C\left(15,4\right)}\\ & =& \frac{6·24}{15·14·13·12}\\ & =& \frac{2}{455}\end{array}$