A ratio is a comparison of numbers or quantities.

The illustration below shows a set of marbles. There are two red marbles and four blue marbles in the set.

The ratio of red marbles to blue marbles could be expressed in the following ways:

• 2 to 4
• 2:4

Note that the ratio 2 to 4 is said to be equivalent to the ratio 1 to 2, that is 2:4 = 1:2.

The illustration below shows three plates, each with four slices of pizza.

The ratio of slices of pizza to plates could be expressed in the following ways:

• 12 to 3
• 12:3

Note also that ratios can have units in each part of a ratio. Here the ratio represents the number of slices of pizza to the number of plates.

The illustration below shows a set of marbles. There are two red marbles, six green marbles and four blue marbles in the set.

The ratio of red marbles to green marbles to blue marbles could be expressed in the following ways:

• 2 to 6 to 4
• 2:6:4

Note that the ratio 1 to 3 to 4 is said to be equivalent to the ratio 2 to 6 to 4, that is 2:6:4 = 1:3:2.