A bag contains 16 red balls and 20 blue balls only. How many white balls must be added to the bag so that the probability of randomly picking a red ball is equal to \(\frac{2}{5}\)
A
4
B
20
C
24
D
40
Verified Correct Answer: A.
Step-by-Step Mathematical Solution
Number of red balls = 16,
Number of blue balls = 20
Let x represent the No of white balls to be added
∴ Total number of balls = 36 + x
Pr( red ball) = \(\frac{ 16}{ 36 + x } = \frac{2}{5}\) (remember that Pr( red ball ) = \frac{2}{5}\))
cross multiplying
2(36 + x) = 80
2x = 80 - 72
2x = 8
x = \(\frac{8}{2}\)
= 4
Source: Myschool Past Questions Archive (Record #30754)