Illustrative Mathematics Grade 7, Unit 5, Lesson 15: Solving Equations with Rational Numbers

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Illustrative Math
Grade 7

Lesson 15: Solving Equations with Rational Numbers

Let’s solve equations that include negative values.

Illustrative Math Unit 7.5, Lesson 15 (printable worksheets)

Lesson 15 Summary

The following diagram shows how to solve equations that include rational numbers and have rational solutions.

Lesson 15.1 Number Talk: Opposites and Reciprocals

The variables a through h all represent different numbers. Mentally find numbers that make each equation true.
3/5 · 5/3 = a
7 · b = 1
c · d = 1
-6 + 6 = e
11 + f = 0
g + h = 0

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Lesson 15.2 Match Solutions

Match each equation to a value that makes it true by dragging the answer to the corresponding equation. Be prepared to explain your reasoning.
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Lesson 15.3 Trip to the Mountains

The Hiking Club is on a trip to hike up a mountain.

1. The members increased their elevation 290 feet during their hike this morning. Now they are at an elevation of 450 feet.
   a. Explain how to find their elevation before the hike.
   b. Han says the equation describes the situation. What does the variable represent?
   c. Han says that he can rewrite his equation as to solve for . Compare Han’s strategy to your strategy for finding the beginning elevation.
   
2. The temperature fell 4 degrees in the last hour. Now it is 21 degrees. Write and solve an equation to find the temperature it was 1 hour ago.
   
3. There are 3 times as many students participating in the hiking trip this year than last year. There are 42 students on the trip this year.
   a. Explain how to find the number of students that came on the hiking trip last year.
   b. Mai says the equation 3s = 42 describes the situation. What does the variable represent?
   c. Mai says that she can rewrite her equation as to solve for s = 1/3 · 42. Compare Mai’s strategy to your strategy for finding the number of students on last year’s trip.
   
4. The cost of the hiking trip this year is 2/3 of the cost of last year’s trip. This year’s trip cost $32. Write and solve an equation to find the cost of last year’s trip.
   

Are you ready for more?

A number line is shown below. The numbers 0 and 1 are marked on the line, as are two other rational numbers a and b.
Decide which of the following numbers are positive and which are negative

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Lesson 15.4 Card Sort: Matching Inverses

Your teacher will give you a set of cards with numbers on them.

1. Match numbers with their additive inverses.
2. Next, match numbers with their multiplicative inverses.
3. What do you notice about the numbers and their inverses?

Lesson 15 Practice Problems

1. Solve.
   a. 2/5 t = 6
   b. -4.5 = a - 8
   c. 1/2 + p = -3
   d. 12 = x · 3
   e. -12 = -3y
   
2. Evaluate each expression if x is 2/5, y is -4, and z is -0.2.
   a. x + y
   b. 2x - z
   c. x + y + z
   d. y · x
   
3. Match each equation to a step that will help solve the equation.
   
4. a. Write an equation where a number is added to a variable, and a solution is -8.
   b. Write an equation where a number is multiplied by a variable, and a solution is
   
5. The markings on the number line are evenly spaced. Label the other markings on the number line.
   
6. In 2012, James Cameron descended to the bottom of Challenger Deep in the Marianas Trench; the deepest point in the ocean. The vessel he rode in was called DeepSea Challenger.
   Challenger Deep is 35,814 feet deep at its lowest point
   a. DeepSea Challenger’s descent was a change in depth of (-4) feet per second. We can use the equation y = -4x to model this relationship, where y is the depth and x is the time in seconds that have passed. How many seconds does this model suggest it would take for DeepSea Challenger to reach the bottom?
   b. To end the mission DeepSea Challenger made a one-hour ascent to the surface. How many seconds is this?
   c. The ascent can be modeled by a different proportional relationship y = kx. What is the value of k in this case?
   

The Open Up Resources math curriculum is free to download from the Open Up Resources website and is also available from Illustrative Mathematics.

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