Question
A bag contains 5 red balls and some blue balls. If the probability of drawing a blue ball is double that of a red ball, then the number of blue balls in a bag is:
- 5
- 10
- 15
- 20
Hint:
We have to find the number of blue balls, if the probability of blue ball is double than the probability of red ball. First we will find the total number of balls, for that we will assume the probability of getting red ball to be x and so the probability of getting blue ball is 2x and as the sum of probability of all the possible outcome is 1 so, by equating we will get the probability of getting red ball, further as the no. of red balls is given we can equate possible probability with probability derived. After getting the total number of balls, we can subtract number of red balls from it to get the number of blue ball.
The correct answer is: 10
Let the probability of getting balls be x.
Then the probability of getting blue ball = 2x
As we know the sum of probability of all the possible outcomes is 1.
So, 2x + x = 1
Let total number of probability be P.
The probability of getting red balls =
but as derived probability of getting red ball = x =
So, 
total number of outcome i.e. total no. balls is 15
so, no. blue balls = 15 - 5 =10.
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