Question

A bag contains only red and blue marbles. Jasmine takes one marble at random from the bag. The probability that she takes a red marble is 1 in 5. Jasmine 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?

10 red



3 red



2 red


5red

Answer 1

5 red marbles.

Explanation:

Let the number of red marbles be x , then the number of blue marbles will be 4x and the total number of marbles is 5x. (because Prob(red) = 1/5).

After adding 5 marbles the total number of marbles in the bag is 5x + 5.

The new probability of picking red is 1/3 so we have the equation:

Total of red marbles / total of all marbles = (x + 5) / (5x + 5) and this = 1/3.

(x + 5) / (5x + 5) = 1/3

5x + 5 = 3x + 15

2x = 10

x = 5.

Answer 2

The answer is the last one 5 red