Question

in a box there is 3 green and 6 blue marbles find the no of ways in which 2 green and 5 blue marbles can be drawn out​

Answer 1

You are correct. Another way to compute it:

Let Ai be the event in which the i-th ball you grab is red. For the first ball you choose, you have 3 red marbles out of a total of 18. Therefore,

P(A1)=318.

For the second marble you grab, given that you already took a red one, that is, given A1, you have 2 red balls out of a total of 17, then

P(A2∣A1)=217.

Finally,

P(A1∩A2)=P(A1)P(A2∣A1)=318217=151.

You can use the same reasoning to compute for the purple ones and then add those probabilities.