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?
There were originally in the bag:
5 Red marbles + 20 Blue marbles =25 total.
The probability of withdrawing a Red marble was =5 / 25 = 1 / 5
She put the Red marble back into the bag, then added 5 more Red marbles.
Now she has: original 5 Red marbles + 5 new Red marbles =10 Red marbles + 20 original Blue marbles.
Now, she has 10 Red marbles + 20 Blue merbles = 30 marbles altogether.
The probability of withdrawing a Red marble is: 10 / 30 = 1 / 3