Question

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

The probability of randomly choosing a red marble is 3/8.
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

27

Explanation:

specific probability = number of specific possibilities divided by number of total possibilities.

the number of total possibilities is 72 (we can pull one out of 72).

the probability of pulling a red back is 3/8.

so, to find the actual number of specific possibilities (actual number of red balls in the overall sample size of 72) we need to bring this ratio of 3/8 to an expression of x/72.

what is the factor to turn 8 into 72 ? 9.

8×f = 72

f = 72/8 = 9

to keep the ratio unchanged in its dimension, we need to apply the same factor to 3 to get x.

3/8 × f/f = x/72

3/8 × 9/9 = x/72

3×9 / 72 = x/72

3×9 = x = 27

so, the probability ratio based on actual possibilities (number of balls available) is 27/72

=>

there are 27 red balls in the bag of total 72 balls.