There is a bag filled with 5 blue, 6 red and 2 green marbles. A marble is taken at random from the bag, the colour is noted and then it is replaced.
Another marble is taken at random.
What is the probability of getting 2 greens?

There are 13 balls. The probability of getting a red ball on the first draw is 6/13. Given replacement, the probability of getting a red ball on the second draw is also 6/13. The probability of getting two red balls is 6/13*6/13 or 36/169.