72 : 40 : : ? : 30*

This is a proportion problem. The relationship between the first two numbers is that 72 is 1.8 times 40. So, to find the unknown number, you need to find a number that is 1.8 times 30.

Here are the steps:

1. Identify the relationship between the first two numbers. In this case, 72 is 1.8 times 40.

2. Apply the same relationship to the third number to find the fourth number. So, you need to find a number that is 1.8 times 30.

3. Multiply 1.8 by 30.

1.8 * 30 = 54

So, the unknown number is 54.

Therefore, 72 : 40 :: 54 : 30.