Question

A bag contain 5 red balls and some blue balls. If the probability of drawing a blue ball from the bag is four time that of a red ball, then find the number of blue balls in the bag.

Answer 1

let number of blue ball = x
Given
Probability(Blue ball)= 2*Probability(Red ball)
(x/(5+x))=2(5/(x+5))
solve x=10

Answer 2

let number of blue balls be x;
(5/(5+x))+4*(5/(5+x))=1
x=20 is you answer