Problem: Find the difference between two numbers in the ratio 7:3 if their sum is 30.

Solution:
To solve the problem, we can use the following steps:

Step 1: Assign variables to the two numbers in the ratio.
Let the two numbers be 7x and 3x.

Step 2: Use the given information to form an equation.
The problem states that the sum of the two numbers is 30, so we can write the equation:
7x + 3x = 30

Step 3: Solve for x.
Combining like terms, we get:
10x = 30
Dividing both sides by 10, we get:
x = 3

Step 4: Find the two numbers.
Using the value of x, we can find the two numbers in the ratio:
7x = 7(3) = 21
3x = 3(3) = 9

Therefore, the two numbers are 21 and 9.

Step 5: Find the difference between the two numbers.
The difference between the two numbers is:
21 - 9 = 12

Therefore, the difference between the two numbers in the ratio 7:3 if their sum is 30 is 12.

Conclusion: The difference between two numbers in ratio 7:3 when their sum is 30 is 12.