Math 68
Which Is the Best Buy?
Eric has opened a new video store in town. He is advertising $1.50 for each video rented. He claims that his videos are the cheapest. However, Sarah’s Short Stop says their videos are the better buy. For just an $18 membership fee, videos only cost $1 for each. Use mathematics to decide which video rental is the best buy.
Suggested Grade Span:
Grades 68
Grade(s) in Which Task Was Piloted:
Grade 78
Alternative Versions of Task
More Accessible Version:
Eric has opened a new video store in town. He is advertising $1.50 for each video rented. He claims that his videos are the cheapest. However, Sarah’s Short Stop says their videos are the better buy. For just an $18 membership fee, videos only cost $1 for each. If a customer wants to rent 2 videos a month for 1 full year, use mathematics to decide which video rental is the best buy.
More Challenging Version:
The original version, and …
Research the rates at a video store near you. How does its rates compare to those of Eric’s and Sarah’s?
NCTM Content Standards and Evidence
Algebra Standard for Grades 68
Instructional programs from Pre Kindergarten through grade 12 should enable students to ...
 Understand patterns, relations, and functions
 Use mathematical models to represent and understand quantitative relationships
 NCTM Evidence: Represent, analyze, and generalize a variety of patterns with tables, graphs, words, and, when possible, symbolic rules
 NCTM Evidence:Relate and compare different forms of representation for a relationship
 NCTM Evidence:Model and solve contextualized problems using various representations, such as graphs, tables, and equations
 Exemplars Task Specific Evidence: This task requires students to find the breakeven point of two sets of data.
Time/Context/Qualifiers/Tip(s) From Piloting Teacher
This task takes one to two 45minute class periods to complete.
Links
This task could link to other “better buy” tasks that require finding the breakeven point to solve, such as the “Calculating Calculator Costs” also featured in this issue of Exemplars.
Common Strategies Used to Solve This Task
Most students will start with a chart, translate the data to a double line graph, and use the intersection of the two graphs to determine the breakeven point.
Possible Solutions
Original Version:
# of videos  cost at sarah's  Cost at eric's 
1  $19.00  $1.50 
2  $20.00  $3.00 
3  $21.00  $4.50 
4  $22.00  $6.00 
5  $23.00  $7.50 
6  $24.00  $9.00 
7  $25.00  $10.50 
8  $26.00  $12.00 
9  $27.00  $13.50 
10  $28.00  $15.00 
11  $29.00  $16.50 
12  $30.00  $18.00 
13  $31.00  $19.50 
14  $32.00  $21.00 
15  $33.00  $22.50 
16  $34.00  $24.00 
17  $35.00  $25.50 
18  $36.00  $27.00 
19  $37.00  $28.50 
20  $38.00  $30.00 
21  $39.00  $31.50 
22  $40.00  $33.00 
23  $41.00  $34.50 
24  $42.00  $36.00 
25  $43.00  $37.50 
26  $44.00  $39.00 
27  $45.00  $40.50 
28  $46.00  $42.00 
29  $47.00  $43.50 
30  $48.00  $45.00 
31  $49.00  $46.50 
32  $50.00  $48.00 
33  $51.00  $49.50 
34  $52.00  $51.00 
35  $53.00  $52.50 
36  $54.00  $54.00 
37  $55.00  $55.50 
38  $56.00  $57.00 
39  $57.00  $58.50 
40  $58.00  $60.00 
41  $59.00  $61.50 
42  $60.00  $63.00 
43  $61.00  $64.50 
44  $62.00  $66.00 
45  $63.00  $67.50 
46  $64.00  $69.00 
47  $65.00  $70.50 
48  $66.00  $72.00 
49  $67.00  $73.50 
36 videos is the break even point, after that Sarah's is the better buy.
The task can also be approached by solving systems of linear equations.
y = 1.5 x
y = x + 18
Use the substitution method:
1.5 x = x + 18
.5 x = 18
x = 36 videos is the breakeven point
More Accessible Version:
# of Videos  cost at Sarah's  cost at eric's 
1  $19.00  $1.50 
2  $20.00  $3.00 
3  $21.00  $4.50 
4  $22.00  $6.00 
5  $23.00  $7.50 
6  $24.00  $9.00 
7  $25.00  $10.50 
8  $26.00  $12.00 
9  $27.00  $13.50 
10  $28.00  $15.00 
11  $29.00  $16.50 
12  $30.00  $18.00 
13  $31.00  $19.50 
14  $32.00  $21.00 
15  $33.00  $22.50 
16  $34.00  $24.00 
17  $35.00  $25.50 
18  $36.00  $27.00 
19  $37.00  $28.50 
20  $38.00  $30.00 
21  $39.00  $31.50 
22  $40.00  $33.00 
23  $41.00  $34.50 
24  $42.00  $36.00 
For 24 videos, Eric has the better buy.
More Challenging Version:
Results will vary depending on the data the student collects on an additional video store.
Task Specific Assessment Notes
General Notes: This task lends itself well to students using math representations to solve the task and communicate the solution.
Task Specific Rubric/Benchmark Descriptors
Click on a level for student example. 


Novice  The student has limited understanding of the task. No progress is made toward a solution. No math language or representations are used. 
Apprentice  Partial understanding is demonstrated. An incorrect conclusion is achieved. Computation or reasoning errors are present. Some math language and representations are used. Some awareness of audience is evident. 
Practitioner  A correct solution is achieved. All work is shown and accurate. Math representations are used as a strategy to solve the task and to communicate the solution. 
Expert  All work is correct, organized, and accurate. Math representations are used as a strategy to solve the task and to communicate the solution. Awareness of audience is intentional. Algebraic notation is used to communicate the solution. 
Novice
Apprentice
Practitioner
Expert