8th Grade Math Prompt

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8th Grade Math Prompt 1
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1. Desiree deposits 1000 in her savings account.
The account pays simple interest at a rate of
5 per year. How much will be in her account
after three years? (Assume that she does not
deposit or withdraw any other amounts during
this time.)

• Make a table to follow the account.
• year in account
• 0 1000
• 1 (5 of 1000) 1000 (50)
  1000 1050
• 2 (5 of 1050) 1050 (52.50)
  1050 1,102.50
• 3 (5 of 1,102.50) 1,102.50
  (55.13) 1,102.50 1,157.63

Answer 1,157.63
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Grading was as follows

• 2 points both procedure answer correct
• 1 point correct procedure or correct answer
• 0 point no correct procedure or answer

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2. A grocery store receives a shipment of 50
gallons of milk. It has 46 gallons left after
2 hours, 22 gallons left after14 hours, and 8
gallons left after 21 hours. Draw a graph of
the quantity of milk left as a function of time
and find the slope of the graph.

• First graph the data.

Milk Left
60 50 40 30 20 10
Gallons
Remember your titles and choose a correct scale!
0 10 20 30 40
50 60
Time
5
Find the slope by choosing two points on the
graph. Remember, slope is rise / run or the
change in y / change in x.
22 46 -24 -2 14 2 12

Milk Left
60 50 40 30 20 10
(2,46)
Gallons
(14,22)
0 10 20 30 40
50 60
Answer Slope is - 2
Time
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Grading was as follows

• 2 points graph and equation were correct
• 1 point graph or equation were correct
• 0 points no correct answers

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3. a. Write an equation of the line with slope 4
and y-intercept 5.

• Since the slope intercept form of a line is
• y mx b
• where m is the slope and b is the
  y-intercept, we can just plug in the given values.

Answer y 4x 5
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b. Write an equation of the line with slope 5
that passes through the point (0,3).

• First fill in the equation with the values you
  already know.
• y 5x b

You need to find the y-intercept or b. Do this
by plugging in the x and y values from the given
point. Then solve for b. 3 5(0) b 3
0 b 3 b
Answer y 5x 3
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c. What is the equation of the following line?
8 6 4 2
2 4 6 8
-8 -6 -4 -2
-2 -4 -6 -8
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I can see the y-intercept is 5.
I need to calculate the slope. Rise / Run or
change in y / change in x. Slope - 2
8 6 4 2
Pick two points. (0,5) and (2,1)
1 - 5 -4 -2 2 0 2
2 4 6 8
-8 -6 -4 -2
-2 -4 -6 -8
Equation of line y -2x 5
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Grading was as follows

• 3 points all three answers correct
• 2 points two of three answers correct
• 1 point one of three answers correct
• 0 points no correct answers