Aptitude

1. 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?

Soln: Since we only pick one ball so we don’t need combination or permutation

Total Balls = 8 + 7 + 6 = 21 balls

Trick: the ball should be neither red nor green, which means it should be blue so total blue balls are 7.

Therefore the probability is 7/21 = 1/3 is the probability.



2. 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?

Soln: Since we pick two balls randomly so we use combination (order doesn't matter)

Total Balls: 2 + 3 + 2 = 5 

Trick: None of the ball drawn are blue, that means we can pick either red/green and total no. of red and green balls are 5. 

Therefor firstly we have to fine the combination of red and green picked randomly (2 balls drawn randomly)

E = Event of drawing 2 balls, none are blue

Let S be the sample space 

n(S) = Number of ways of drawing 2 balls out of 7

Hence the probability is  = 10/21