Suppose you have the variable expression $$\frac{3m2}{8}$$ This expressions means "I'm thinking of a number that is multiplied by 3, then subtracted by 2, and finally divided by 8." Evaluating an expression means replacing the variable with an actual number. For example, if m = 6, then $$\frac{3m2}{8}=\frac{3\cdot~62}{8}=\frac{182}{8}=\frac{16}{8}=2$$ 
Example 1
Martha went to the hobby store and purchased two bags of marbles plus 3 extra marbles. 
We don't know the number of marbles in each bag, so all we can say is that she bought 2x + 3 marbles.
If we are told there are 42 marbles in each of Martha's bags, then we can substitute 42 for the x in the variable expression.
2x + 3 > 2(42) + 3 = 84 + 3 = 87 marbles
Martha bought 87 marbles.
Example 2 Evaluate $2x+3$ if x = 17. 
Solution: Replace the variable with 17 and then simplify. 
Example 3
Jenny was filling bottles from a tank of water. When she had filled four bottles with water, 1 cup of water was left in the tank. How much water is in the tank? 
The variable expression for this situation is 4c + 1, where c represents the number of cups in each bottle.
If c = 6, how many cups of water are in the tank?
Example 4
If a = 3, b = 6, and c = 4 evaluate each of the following expressions.
$\large~ab+c$ 
$\large~\frac{bc}{2}$ 
$\large~\frac{a(b+4)}{c+1}$ 



Question 1 Evaluate $4x6$ for $x=4$. 
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Question 2 Evaluate $6x+2x$ for $x=3$. 
[show answer] 
Question 3 Evaluate $\frac{39}{x}+5y$ for $x=3\mbox{ and }y=5$. 
[show answer] 