A bag contains 56 marbles. each marble is either red, green, or yellow. a marble is randomly removed from the bag and then returned to the bag. the probability that this marble is red is 5/8. the probability that this marble is green is 1/7. how many yellow marbles are in the bag?

The first time I solved this problem, I did it in a way that I had to

mess with changing fractions to common denominators and all that.

Suddenly, however, the clean and easy way to do it dawned on me.

Here it is:

- - There are 56 marble in the bag.

- - The probability of pulling a red marble is 5/8, so

5/8 of the 56 marbles must be red.

5/8 of 56 = 35 red marbles.

- - The probability of pulling a green marble is 1/7,

so 1/7 of the 56 marbles must be green.

1/7 of 56 = 8 green marbles.

- - The red and green marbles make up (35 + 8) = 43

of the 56 marbles in the bag.

- - The rest of them are yellow.

There are (56 - 43) = 13 yellow marbles in the bag.

- - The probability of pulling a yellow marble is 13/56. (about 23.2%)

No messy fractions at all!

That's the way I like it!

1/7 + 5/8 + x = 56

43/56 + x = 56

13 = x