Question

There is a bag with only red marbles and blue marbles.

The probability of randomly choosing a blue marble is
5/12
.
There are 72 marbles in total in the bag and each is equally likely to be chosen.
Work out how many red marbles there must be.

Answer 1

42 reds.

Explanation:

There are 72 marbles in the bag.

P(Red) = 1 - Blue = 1 - 5/12 = 7/12

P(red) = 7/12 * 72 = 42

If 1 - 5/12 confuses you, then find the blue first

5/12 * 72 = 5 * 72/12 = 5 * 6 = 30

The rest must be red.

72 - 30 = 42