Answer: 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
