# Evaluating algebraic expressions

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##### Intros
###### Lessons
1. i) What are variables?

ii) What is an expression?

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##### Examples
###### Lessons
1. Below we have used ponds and tadpoles to model an expression. Write the expression and use the variable x to represent the unknown number of tadpoles in each pond.

2. Mary bought $s$ packages of stickers, and there are 10 stickers in each package. Write an expression to show how many stickers Mary bought.
1. Write an expression for each phrase. Then, evaluate the expression.
1. 10 pounds lighter than Molly (m), when m=100
2. 22 years older than Tracy (t), when t = 10.
3. 17 less than 5 times a number (n), when n = 7
2. Evaluate the following expression, if $x = 7$ and $y = 9$
1. $6x-y+5$
2. $\frac{2}{3}x+\frac{1}{6}y-1$
3. $0.5x-0.1+1.3y$
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##### Practice
###### Topic Notes
In this section, we are given models of cups and counters and asked to write the expressions represented by these models. We then use the variable x to represent the number of counters in each cup. We were first taught how to write expressions and solve for variables in a previous section. In this section, we are also asked to draw cup and counter models of given expressions. Finally, we are asked to write expressions for given phrases and evaluate the expressions using our variables.