An urn contains 2 red, 3 green and 2 blue balls. If 2 balls are drawn at random, find the probability that no ball is blue.

Answer: Option B

Total number of balls = (2 + 3 + 2) = 7
Let, E be the event of drawing 2 non-blue balls.
Then, n (E) = $${}^5{C}_4=\frac{5\times 4}{2\times 1}$$   = 10
And, n (S) = $${}^7{C}_2=\frac{7\times 6}{2\times 1}$$   = 21
$$\therefore P(E)=\frac{n(E)}{n(S)}=\frac{10}{21}$$