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Probability - General Aptitude

Q1:

A bag contains 2 red, 3 green and 2 blue balls. Two balls are drawn at random. What is the probability that none of the balls drawn is blue?

A
10
21

B
11
21

C
2
7

D
5
7

ANS:A -

10
21

Total number of balls = (2 + 3 + 2) = 7. Let S be the sample space.

Then, n(S) = Number of ways of drawing 2 balls out of 7
= 7C2 `
= (7 x 6)
(2 x 1)
= 21.
Let E = Event of drawing 2 balls, none of which is blue.
n(E) = Number of ways of drawing 2 balls out of (2 + 3) balls.
= 5C2
= (5 x 4)
(2 x 1)
= 10.
P(E) = n(E) = 10 .
n(S) 21