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?
Since the first probability is 1/5, the number of read marbles was either 1 (if there were 5 marbles total), 2 (if there were 10 total), 3 (15 total), 4 (20 total), 5 (25 total), etc.. When you add 5 red marbles to the bag, you are increasing each of those fractions by 5/5. Therefore 1/5 would turn into 6/10 or 3/5. That's not our last probability so we keep going. 2/10 would turn into 7/15. Nope. 3/15 turns into 8/20 or 2/5. Nope. 4/20 turns into 9/25. Nope. 5/25 turns into 10/30 or 1/3. Therefore, the bag started with 5 red marbles out of a total of 25 marbles.
