The ratio is defined as the comparison of two quantities of the same units.

According to proportion, if two sets of given numbers are increasing or decreasing in the same ratio, then the ratios are said to be directly proportional to each other. Proportions are denoted using the symbol "::" or "=".

Explanation: To find the third proportional number to 9 and 12, assign it the variable x. Use the proportion 9 : 12 :: 12 : x. Cross-multiply to get x = 16. Therefore, the third proportional number is 16.