Writing and Evaluating Addition and Subtraction Equations

What is an equation? An equation is a mathematical sentence that has an equal sign. An equation has ONLY one solution. A solution is an answer that can be substituted in for a variable to make an equation true.

We can use inverse operations to write and solve addition and subtraction equations. Inverse operations undo each other. For example, to solve an addition equation, use subtraction. When writing equations, a variable should ALWAYS be paired with a number and an operation! Your variable should NEVER be by itself on one side of the equals sign! Yes: 4 + y = 10 NO!!!! 4 + 6 = y

Subtraction Property of Equality If you subtract the same number from each side of an equation, the two sides remain equal.

Addition Property of Equality If you add the same number from each side of an equation, the two sides remain equal.

x + 3 = 10 x = 7 x + 9.5 = 14.3 x = 4.8 ¼ + x = ¾ x = ½

13 = x + 9 x = 4 x + 9 = 18 x = 9 x + 2/7 = 5/14 x = 1/14

x – 7 = 12 x = 19 x – 17.06 = 13 x = 30.06 3.86 = x – 5.97 x = 9.83

34 = x – 8 x = 42 x – 9.24 = 25.02 x = 15.78 X – 3/18 = 11/18 x = 14/18

Solve the following equations. x + 18.9 = 46.3 27.4 p – 2.12 = 41.73 43.85 4 ½ + p = 7 3/10 2 4/5 5 ¾ - y = 4 ¼ 1 ½

End of today’s lesson

You have 10 minutes left at a parking meter You have 10 minutes left at a parking meter. After you put in a quarter, you have 25 minutes left. Write and solve an equation to find q, the number of minutes a quarter is worth.

You eat 9 strawberries from a carton. There are 12 strawberries left You eat 9 strawberries from a carton. There are 12 strawberries left. How many strawberries were in the carton? Write and solve an equation for the situation.

Your class is planting trees. You have planted 17 trees Your class is planting trees. You have planted 17 trees. You all have 14 tress left to plant. Write and solve an equation to find the number of trees the class started with.

This month you have talked 153 minutes on your cell phone This month you have talked 153 minutes on your cell phone. You can use 250 minutes before you have to pay extra fees. Write and solve an equation to find the number of minutes, m, before you have to pay extra fees.

You are delivering newspapers on your paper route You are delivering newspapers on your paper route. You have to deliver to 8 more houses. You have already delivered to 21 houses. How many houses are on your route? Write and solve an equation for the situation.

A video game has 16 levels. You have reached level 5 A video game has 16 levels. You have reached level 5. Write and solve an equation to find the number of levels you have left.

You have lost track of how many softball games you have played this season. You know your team won 12 games. Your friend reminds you that your team lost 9 games. How many games did your team play? Write and solve an equation for the situation. You buy a shirt for $17.45. You give the clerk a $20 bill. How much change do you get back? Write and solve an equation for the given situation.