standard_description: Add and subtract fractions with unlike denominators (including mixed numbers) by replacing given fractions with equivalent fractions in such a way as to produce an equivalent sum or difference of fractions with like denominators. For example, 2/3 + 5/4 = 8/12 + 15/12 = 23/12. (In general, a/b + c/d = (ad + bc)/bd.)
problem_activity: Activity
In the previous activity students looked at 9 different equations. In this activity, each student focuses on just one of those equations to learn about the real-world situation it represents. They interpret the variables and values in terms of the situation (MP7). Adding to the table they saw in the previous activity gives students an opportunity to practice both evaluating expressions and solving equations.
This activity also asks students to create a graph of their assigned relationship on graph paper. In previous activities, students were given a grid with the axes already labeled and numbered whenever they were asked to create a graph of a relationship. Here they must decide for themselves how to scale each axis.
Launch
Decide whether you want to assign the equations or allow students to select which equation they will work with. Make sure there is at least one student working with each equation.
Give students 5–10 minutes of quiet work time followed by partner and whole-class discussion. If there is more than one student working with the same equation, consider letting them share their answers and reasoning with each other, after they have had some time to work on their own.
Action and Expression: Internalize Executive Functions.
Chunk this task into more manageable parts. Check in with students within the first 2-3 minutes of work time. Look for students who need additional support deciding how to scale each axis for table. Invite students to share strategies they have used so far, as well as any questions they have before continuing.
Supports accessibility for: Memory; Organization
Student Facing
The equations in the previous activity represent situations.
\(S - 2 = T\)
where
\(S\)
is the number of sides on a polygon and
\(T\)
is the number of triangles you can draw inside it (from one vertex to the others, without overlapping)
\(G = J + 13\)
where
\(G\)
is a day in the Gregorian calendar and
\(J\)
is the same day in the Julian calendar
\(P = I - 47.50\)
where
\(I\)
is the amount of income and
\(P\)
is the profit after
$
47.50 in expenses
\(C + 273.15 = K\)
where
\(C\)
is a temperature in degrees Celsius and
\(K\)
is the same temperature in Kelvin
\(e = 6s\)
where
\(e\)
is the total edge length of a regular tetrahedron and
\(s\)
is the length of one side
\(m = 8.96V\)
where
\(V\)
is the volume of a piece of copper and
\(m\)
is its mass
\(y = \frac{1}{12}x\)
where
\(x\)
is the number of eggs and
\(y\)
is how many dozens that makes
\(t = \frac{d}{2.5}\)
where
\(t\)
is the amount of time it takes in seconds to jog a distance of
\(d\)
meters at a constant speed of 2.5 meters per second
\(g = 28.35z\)
where
\(g\)
is the mass in grams and
\(z\)
is the same amount in ounces
Your teacher will assign you one of these equations to examine more closely.
Rewrite your equation using words. Use words like product, sum, difference, quotient, and term.
In the previous activity, you matched equations and tables. Copy the values from the table that matched your assigned equation into the first 3 rows of this table. Make sure to label what each column represents.
| independent variable:                             ________________________ | dependent variable:                             ________________________ |
|----------------------------------------------------------------------------|--------------------------------------------------------------------------|
|                                                                            |                                                                          |
|                                                                            |                                                                          |
|                                                                            |                                                                          |
|                                     60                                     |                                                                          |
|                                                                            |                                   300                                    |
Select one of the first 3 rows of the table and explain what those values mean in this situation.
Use your equation to find the values that complete the last 2 rows of the table. Explain your reasoning.
On graph paper, create a graph that represents this relationship. Make sure to label your axes.
Student Response
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Anticipated Misconceptions
Some students may struggle to understand what each step is asking them to do. Consider preparing an example to share with them, such as this for the equation
\(2.54i = c\)
, and point out which part of the example corresponds to the question they are on.
The number of centimeters is the product of the number of inches and 2.54.
| independent variable:                       length (inches) | dependent variable:                       length (centimeters) |
|-------------------------------------------------------------|----------------------------------------------------------------|
|                              5                              |                              12.7                              |
|                             36                              |                             91.44                              |
|                             75                              |                             190.5                              |
|                             60                              |                                                                |
|                                                             |                              300                               |
If something is 36 inches long, this is equivalent to 91.44 when measured in centimeters.
In the table above, fill in 152.4 in the right column, next to the 60, because
\(2.54 \boldcdot 60 = 152.4\)
. Fill in 118.11 in the left column, next to the 300, because
\(2.54 \boldcdot 118.11 \approx 300\)
A graph with “length (inches)” on the horizontal axis, “length (centimeters)” on the vertical axis, and points at
\((5, 12.7), (36, 91.44), (75, 190.5), (60, 152.4)\)
, and
\((118.11, 300)\)
Some students may struggle with setting up their graph from scratch on graph paper. Prompt them to think about the maximum value they want to represent on each axis and what number they could count by to get to that maximum value in the amount of space they have.
Activity Synthesis
Continue to the next activity. The discussion of these relationships occurs after students have created their visual displays.


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