Question

A bag contains only red and blue marbles. Yasmine takes one marble at random from the bag. The probability that she takes a red marble is 1 in 5. Yasmine returns the marble to the bag and adds five more red marbles to the bag. The probability that she takes one red marble at random is now 1 in 3. How many red marbles were originally in the bag?

Answer 1

R / (r + g) = 1/5 === 5r = r + g === 4r = g 

(r + 5) / (r + 5 + g) = 1/3 --- 3r + 15 = r + 5 + g 
2r = g - 10 ... 4r = 2g - 20 
g = 2g - 20 
g = 20 .... 4r = g ... r = 5 

20 green and 5 red