Proportionality and Finding the Missing Number
To determine the missing number in a proportion, we can apply the concept of proportionality. In this case, we have the numbers 12, 16, x, and 20 that are in proportion.

Using the Proportionality Rule
The rule for proportionality states that in a proportion, the product of the means is equal to the product of the extremes. Therefore, we can set up the following equation based on the given numbers:
12 * 20 = 16 * x

Solving for x
By solving the above equation, we can find the value of x:
240 = 16x
x = 240 / 16
x = 15
Therefore, the missing number in the proportion 12, 16, x, 20 is 15. By applying the concept of proportionality and using the rule of means and extremes, we were able to find the value of x.