Short Answer 
The process involves identifying variables in two rows of numbers, exploring mathematical relationships to discover a consistent equation, and applying that equation to find a missing number in a new row. Ultimately, using the equation derived, the missing number is determined to be 2.
Step 1: Identify the Rows and Variables
First, you need to clearly define the sets of numbers in each row. For our example, there are two rows with the following numbers:
- Row 1: 9 (a), 1 (b), 6 (c), 4 (d)
- Row 2: 4 (a), 5 (b), 7 (c), 2 (d)
Recognizing these variables is crucial for applying the correct mathematical operations to discover the underlying patterns that could lead to the solution.
Step 2: Explore Mathematical Relationships
Next, you need to look for patterns by trying various algebraic operations on the identified variables (a, b, c, and d). The goal is to establish a consistent equation that relates these numbers:
- For Row 1, attempts like (a + b – c = d) yield (9 + 1 – 6 = 4), which is correct.
- Replicate similar equations for Row 2, (4 + 5 – 7 = 2), confirming that the pattern holds.
Ultimately, this step helps uncover the fundamental equation applicable to both rows.
Step 3: Apply the Pattern to Find the Missing Number
Finally, use the established pattern to solve for the missing number in a new row. For Row 3, with values 7, 8, and an unknown represented by “?” leading to 13, apply the previously determined equation:
- Set up the equation: (7 + 8 – ? = 13).
- Calculate to find the value of “?” by isolating it: (15 – ? = 13).
- Solve to discover that ? equals 2.
This systematically concludes that the missing number that completes the pattern is 2.