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Probability Questions and Answers
Probability - Aptitude
16.A box contains 5 green, 4 yellow and 3 white marbles.Three marbles are drawn at random.What is the probability that they are not of the same colour?
x
Option: D
Explanation
n(S) = number of ways of drawing 3 marbles out of 12
n(S) = 12C3 = (12 * 11 * 10)/(3 * 2 * 1) = 220
E = event of drawing (3 marbles out of 5) or (3 marbles out of 4) or (3 marbles out of 3)
n(E) = 5C3 + 4C3 + 3C3=5C2 + 4C1 + 1 = ((5 * 4)/(2 * 1)) + 4 + 1 = 15
P(E) = n(E)/n(S) = 15/220 = 3/44
17.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?
x
Option: A
Explanation
n(S) = number of ways of drawing 2 balls out of 7
n(S) = 7C2 = (7 * 6)/(2 * 1) = 21
n(E) = number of ways of drawing 2 balls out of (2 + 3) balls
n(E) = 5C2 = (5 * 4)/(2 * 1) = 10
P(E) = n(E)/n(S) = 10/21
18.In a box, there are 8 red, 7 blue and 6 green balls. One ball is picked up randomly. What is the probability that it is neither red nor green?
x
Option: B
Explanation
n(S) = 21 balls
E = event that the ball drawn is red
n(E) = 8 red balls
P(E) = n(E)/n(S) = 8/21
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