A jar contains red and green balls. Someone will randomly selects a ball with the number points earned or lost. There are four red balls, each labelled +3 points . How many green balls labelled -2 points would be required for this to be fair game ?

Answer 1

total number of balls ---- x
number of reds = 4
number of greens = x-4

prob(red) = 4/x
prob(green) = (x-4)/x

3(4/x) - 2(x-4)/x = 0
times x
12 -2x+ 8 = 0
x = 10

So there should be 6 green balls

check:
expected value = 3(4/10) - 2(6/10)
= 12/10 - 12/10 = 0

Answer 2

Thx