1. Warm Up
Solve.
1. x + 12 = 35
2. 8x = 120
= 7
4. –34 = y + 56
x = 23
x = 15 y9
y = 63
y = –90

2. Recall that two-step equations contain two operations, and therefore, require two inverse operations to solve. Before solving, ask yourself, “What is being done to the variable, and in what order?” One method to solve the equation is to work backward to undo the operations.

3. Additional Example 2A: Solving Two-Step Equations
Solve = 22.
Use fraction operations.
+ 7 = 22 n3
Since 7 is added to ,
subtract 7 from both sides to undo the addition. n3
+ 7 – 7 = 22 – 7
n 3
= 15 n3
3  = 3  15 n3
Since n is divided by 3, multiply both sides by 3.
n = 45

4. Additional Example 2B: Solving Two-Step Equations
y – 4 3
Solve = 9.
Multiply both sides of the equation by the denominator.
= 9
y – 4 3
= 9
y – 4 3
(3) (3)
Multiply both sides by the denominator.
y – 4 = 27
Since 4 is subtracted from y, add 4 to both sides to undo the subtraction.
y = 31

5. Partner Share! Example 2A
Solve = 18.
Use fraction operations.
+ 8 = 18 n4
Since 8 is added to ,
subtract 8 from both sides to undo the addition. n4
+ 8 – 8 = 18 – 8
n 4
= 10 n4
4  = 4  10 n4
Since n is divided by 4, multiply both sides by 4.
n = 40

6. Partner Share! Example 2B
y – 7 2
Solve = 7.
Multiply both sides of the equation by the denominator.
= 7
y – 7 2
= 7
y – 7 2
(2) (2)
Multiply both sides by the denominator.
y – 7 = 14
Hiding Slide 18
Since 7 is subtracted from y, add 7 to both sides to undo the subtraction.
y = 21

7. Additional Example 1: Problem Solving Application
The mechanic’s bill to repair Mr. Wong’s car was $ The mechanic charges $45.50 an hour for labor, and the parts that were used cost $ How many hours did the mechanic work on the car?

8. Understand the Problem
Additional Example 1 Continued 1
Understand the Problem
List the important information:
The answer is the number of hours the mechanic worked on the car.
The parts cost $
The labor cost $45.50 per hour.
The total bill was $
Let h represent the hours the mechanic worked.
Total bill = Parts Labor
= h

9. Additional Example 1 Continued2
Make a Plan
Think: First the variable is multiplied by 45.50, and then is added to the result. Work backward to solve the equation. Undo the operations in reverse order: First subtract from both sides of the equation, and then divide both sides of the new equation by

10. Additional Example 1 Continued
Solve 3
Since is added to both sides, subtract from both sides.
= h
– –443.75
= h
Since h is multiplied by 45.50, divide both sides by h =
4.6 = h
The mechanic worked for 4.6 hours on Mr. Wong’s car.

11. Additional Example 1 Continued
Look Back 4
You can use a table to decide whether your answer is reasonable.
Hours
Labor
Parts
Total Cost 1
$45.50
$443.75
$489.25 2
$91.00
$534.75 3
$136.50
$580.25 4
$182.00
$625.75 5
$227.50
$671.25
4.6 hours is a reasonable answer.

12. Partner Share! Example 1
The mechanic’s bill to repair your car was $850. The mechanic charges $35 an hour for labor, and the parts that were used cost $275. How many hours did the mechanic work on your car?
Hiding Slide 10

13. Understand the Problem
Partner Share! Example 1 Continued 1
Understand the Problem
List the important information:
The answer is the number of hours the mechanic worked on your car.
The parts cost $275.
The labor cost $35 per hour.
The total bill was $850.
Hiding Slide 11
Let h represent the hours the mechanic worked.
Total bill = Parts + Labor
850 = h

14. Check It Out! Example 1 Continued2
Make a Plan
Think: First the variable is multiplied by 35, and then 275 is added to the result. Work backward to solve the equation. Undo the operations in reverse order: First subtract 275 from both sides of the equation, and then divide both sides of the new equation by 35.
Hiding Slide 12

15. Partner Share! Example 1 Continued
Solve 3
Since 275 is added to both sides, subtract 275 from both sides.
850 = h
–275 –275
575 = h
Since h is multiplied by 35, divide both sides by 35. h =
16.4  h
Hiding Slide 13
The mechanic worked for about 16.4 hours on your car.

16. Partner Share! Example 1 Continued
Look Back 4
You can use a table to decide whether your answer is reasonable.
Hours
Labor
Parts
Total Cost 13
$455
$275
$730 14
$490
$765 15
$525
$800 16
$560
$835 17
$595
$870
Hiding Slide 14
16.4 hours is a reasonable answer.

17. Lesson Review!
Solve.
1. – x – 3 = 10
2. 7y = –26.12
3. –8.3 = –3.5x
= 3.1
5. The cost for a new cell phone plan is $39 per month plus a one-time start-up fee of $78. If you are charged $ , how many months will the contract last? 1 9
x = –117
y = –7.4
x = 6.2 y
y = 29.1
24 months