When evaluating proportions, we write the ratios in fractional form and then we

set the cross product equal.

Explanation.

A proportion is the equality of two ratios.

For example:

1        6
--- =
3       18

You may use the proportions to find the value of an unknown quanity

For example

1        x
--- = 
3       24

So, if you want to solve for the unknown, the next step isto set the cross product equal:

1*24 = 3*x

=> 24 = 3x

From which you can obtain: x = 24 / 3 = 8