In a bag of marbles, 3/5 of them are blue and the rest are red. If the number of red marbles is doubled and the number of blue marbles stays the same, what fraction of the marbles will be red?

Answer 1

Since it is a question of fractions, you can assume any reasonable number and work with the numbers, it's easier.

Say we have 10 marbles, 3/5 of 10=6 are blue, and the rest (4) are red.

If we double the red marbles, we have 8, and the total becomes 8+6=14.
The fraction of red will then be 8/14=4/7.

Try the same approach using 20 marbles to start and see if you get the same answer.

Answer 2

If you would like to solve it using fractions, it is the same approach.

Blue: 3/5
Red: 1-3/5=2/5
Double reds: 4/5
Total : 3/5+4/5=7/5
Fraction of red: (4/5)/(7/5) = 4/7

Answer 3

In a bag are only red and white marbles. The probability of choosing a red is 3/5. The probability of choosing 2 reds without replacement is 3/10 How many white marbles are in the bag?