The Substitution Property of Equality allows us to swap or substitute equivalent quantities in expressions and equations. Let's take a look at a basic example of substitution.

Substitute 3 in for x, because they are equal  

Evaluate 2(3)  

Add 6 and 3  

Our Solution 
Sometimes, we are given an expression for the variable, rather than a single value. We can still use the Substitution Property of Equality to simplify expressions and solve equations. Most often, this requires distribution after substituting, in order to simplify the equation or expression. This is illustrated in the example below:

Substitute 3a2 in for x, because they are equal  

Distribute (.05) into (3a2)  

Combine like terms 1 and 12  

Our Solution 
Let's apply the concept of substitution to solve an equation. Suppose you sell gift bags from a kiosk at a local strip mall. Each gift bag costs $7, and you received $15 in tips for the day. We can represent your profit with the equation: , where R is revenue, and x is the number of gift bags sold. (7 is multiplied by x to represent revenue from sales, and 15 is added to account for the tips.)
You figure that you averaged 8 sales per hour, and at the end of the day, a customer bought 10 of them for a party she is attending. We can represent the number of gift bags sold by the equation , where x is the number of gift bags sold, and t is time in hours. (8 is multiplied by t to represent 8 bags sold each hour, and we add 10 to account for the customer who bought 10 for her party.)
Let's take a look at our equations:
How long did it take to generate $253 in revenue? Notice that we can substitute $253 in for R, but we want to solve for t, time. One method would be to solve for x, and then substitute that value in order to solve for t. Another method involves making all necessary algebraic substitutions first, and then solving a simplified equation.

Substitute in for x, because they are equal  

Distribute 7 into  

Combine like terms 70 and 15  

Subtract 85 from both sides  

Divide by 56  

Our Solution 
This means that $253 was generated after 3 hours of selling gift bags. Not bad!