Given Proportion:
- 12, 16, *, 20 are in proportion

Finding the Missing Value:
- To find the missing value (*), we need to determine the common ratio between consecutive numbers in the proportion.
- The common ratio can be calculated by dividing any number by its previous number in the proportion.
- In this case, the common ratio would be 16/12 = 1.33 and 20/16 = 1.25.

Calculating the Missing Value:
- To find the missing value (*), we can use the common ratio calculated above.
- Since the missing value is between 16 and 20, we can calculate the missing value by multiplying 16 by the common ratio.
- Therefore, 16 * 1.25 = 20. The missing value is 20.

Verification:
- To verify if 12, 16, 20 are in proportion, we can check if the ratios between consecutive numbers are equal.
- The ratio 16/12 = 1.33 and 20/16 = 1.25. Since these ratios are not equal, the numbers 12, 16, *, 20 are not in proportion.

Conclusion:
- The missing value in the proportion 12, 16, *, 20 is 20.