Sometimes, we are given a known quantity for a variable. In the earlier example from last section, I might discover that Peter's monthly salary is \$600, or that your book weighs 12 lbs.

To evaluate an algebraic expression, plug in the known quantity for the variable and evaluate the resulting expression. For example, given the known quantities described above, s + 200 = 600 + 200 = 800 and w + 5 = 3×12 + 5 = 36 + 5 = 41.

You probably have been evaluating algebraic expressions without even knowing it. Every time you plug a number into a formula, like the formula for perimeter of a square, p = 4×s, you are evaluating an expression.

Example 1: What is the perimeter of a square with side length 1? 5? 2.5?
a) p = 4×s = 4×1 = 4
b) p = 4×s = 4×5 = 20
c) p = 4×s = 4×2.5 = 10

Example 2: What is s + 200 if s = 1, 800? If s = 15? If s = - 100?
a) s + 200 = 1, 800 + 200 = 2, 000
b) s + 200 = 15 + 200 = 215
c) s + 200 = - 100 + 200 = 100

Example 3: What is w + 5 if w = 5? If w = 10? If w = 0?
a) w + 5 = 3×5 + 5 = 15 + 5 = 20
b) w + 5 = 3×10 + 5 = 30 + 5 = 35
c) w + 5 = 3×0 + 5 = 0 + 5 = 5