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
Correct Answer: Option A
Explanation
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
2(36 + x) = 80
= 2x + 80 – 72
= 8
x = (frac{8}{2})
= 4