Red balls

In the bag there are 3 red, 12 blue and 8 green balls. How many red balls we must be attached to the bag if we want the probability of pulling out the red balls was 20%?

Correct result:

n =  2

Solution:

(n+3) / (3+12+8+n) = 20/100

(3+12+8+n) • ( (n+3) / (3+12+8+n) ) = (3+12+8+n) • ( 20/100 )

80n = 160

n = 2

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