Equivalent expressions are always equal to each other. If the expressions have variables, they are equal whenever the same value is used for the variable in each expression.

For example,  is equivalent to .

• When  is 3, both expressions equal 21.
• When  is 10, both expressions equal 70.
• When is any other number, both expressions still have equal value.

An inequality is a statement that compares two values or expressions using symbols such as “” or “”. 

• The symbol “” means “is greater than.”
• The symbol “” means “is less than.”

For example, the inequality  tells us that -3 is greater than -7.

A solution to an equation is a number that can be used in place of the variable to make the equation true.

• The solution to the equation is 7, because it is true that .
• The solution to is not 9, because does not equal 8. 

A solution to an inequality is a number that can be used in place of the variable to make the inequality true.

• A solution to the inequality is 5, because it is true that .
• Some other solutions to this inequality are 9.9, 0, and -4.

A variable is a letter that represents a number. Different numbers can be chosen for the value of the variable.

In the expression , the variable is .

• If the value of  is 3, then , because .
• If the value of  is 6, then , because .