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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?

 Dec 17, 2019
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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

 Dec 17, 2019

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