Question

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

The probability of randomly choosing a red marble is 7/9.
There are 45 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

There must be 35 red marbles.

Explanation:

This question can be solved using a rule of three.

A probability is the number of desired outcomes divided by the number of total outcomes.

The probability of randomly choosing a red marble is 7/9.

This means that for each set of 9 marbles, 7 must be red.

There are 45 marbles in total in the bag and each is equally likely to be chosen. Work out how many red marbles there must be.

How many red marbles out of 45?

7 red - 9 marbles

x red - 45 marbles

`9x = 45*7`

Simplifying by 9

`x = 5*7`

`x = 35`

There must be 35 red marbles.