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Substitution in algebra is like a fun game of finding and replacing numbers. It's one of the first steps you'll take when working with equations containing variables, like 'x' or 'y'. When you're given a problem with variables, you're often told what specific numbers to use for those variables. In our example, we're working with the expression \( 20 - 4x \) and need to substitute \( x = 4 \).

• Find the variable in the expression.
• Replace the variable with its given value.

After substitution, the expression \( 20 - 4x \) becomes \( 20 - 4 \, \times \, 4 \). This step sets us up to tackle the arithmetic in the problem. By substituting, we turn abstract algebra into concrete numbers that we can easily manage. Substitution makes complex problems simpler by giving us actual values to work with. This 'plug-and-play' method is foundational in learning and mastering algebraic solutions.

Once you've substituted the variables with their respective numbers, it's time to perform multiplication. Multiplying numbers is a critical step in many algebraic expressions. Here, we need to handle \( 4 \, \times \, 4 \).

• Perform the calculation, \( 4 \, \times \, 4 = 16 \).

Multiplication follows specific rules, such as the commutative property, where changing the order of the numbers doesn't affect the answer (\( a \, \times \, b = b \, \times \, a \)). Applying correct multiplication simplifies expressions, which means bringing numbers down to a form that is easier to deal with. In our exercise, after multiplying, you're left with \( 20 - 16 \), ready for the next step.
This operation of resolving expressions into a single number through multiplication is indispensable in basic algebra. It clears the way for further simplifications or solutions.

Following multiplication, the next step is subtraction. Subtraction is about finding the difference between two numbers, which involves taking away one quantity from another. Our expression after substitution and multiplication is \( 20 - 16 \).

• Subtract \( 16 \) from \( 20 \).
• The result of \( 20 - 16 \) is \( 4 \).

Subtraction is usually the final step in simplifying expressions or solving equations. It tells us what's left after one quantity is removed from another. In algebra, like in our example, subtraction reduces our expression down to a final answer, \( 4 \).
This simple operation might seem straightforward, but it's critical for balancing equations, solving problems, and understanding the relationships between numbers. Subtraction completes our journey from a complicated expression with variables to a plain number.