Grade 8 Math Practice Test: Scatter Plots, Lines of Best Fit and Two-Way Tables
- Scatter Plots and Two-Way Tables
- 10 questions
- 15 minutes
This test covers describing patterns in scatter plots, using a line of best fit to make predictions and reading two-way tables with relative frequencies. Tip: compare groups of different sizes with percentages, not with raw counts.
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Question 1
A scatter plot shows that students who practice piano more hours per week tend to make fewer mistakes at their recital. What kind of association is this?
Correct answer: C
When one variable goes up while the other goes down, the association is negative. A positive association would mean more practice and more mistakes.
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Question 2
A line of best fit for test scores y and hours studied x is y = 4.5x + 52. What score does the line predict for 6 hours of study?
Correct answer: C
y = 4.5 · 6 + 52 = 27 + 52 = 79. 56.5 adds 4.5 and 52 without multiplying by 6.
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Question 3
A line of best fit for test scores y and hours studied x is y = 4.5x + 52. What does the slope 4.5 mean?
Correct answer: A
The slope is the change in y for each 1-unit increase in x, so each extra hour predicts about 4.5 more points. The predicted score with no study is the y-intercept, 52.
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Question 4
Which pair of variables would most likely show no association?
Correct answer: A
Shoe size has no link to how well an adult does in math. Height and arm span rise together, driving time and distance rise together, and heating costs fall as the temperature rises (negative association).
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Question 5
Students in an after-school survey Plays a sport No sport Total Grade 7 18 12 30 Grade 8 14 6 20 Total 32 18 50 What percent of the Grade 8 students play a sport?
Correct answer: D
14 of the 20 Grade 8 students play a sport: 14 ÷ 20 = 70%. About 44% is 14 ÷ 32, the share of sport players who are in Grade 8.
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Question 6
Students in an after-school survey Plays a sport No sport Total Grade 7 18 12 30 Grade 8 14 6 20 Total 32 18 50 Of the students who do not play a sport, about what percent are in Grade 7?
Correct answer: D
18 students play no sport, and 12 of them are in Grade 7: 12 ÷ 18 ≈ 67%. 40% is 12 ÷ 30, the share of Grade 7 students who play no sport.
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Question 7
Students in an after-school survey Plays a sport No sport Total Grade 7 18 12 30 Grade 8 14 6 20 Total 32 18 50 Which statement is supported by the table?
Correct answer: D
70% of Grade 8 students (14 of 20) play a sport, compared with 60% of Grade 7 students (18 of 30). In raw numbers, more Grade 7 students play (18 vs. 14), and 32 of 50 is 64%, not half.
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Question 8
A line of best fit on a scatter plot passes through (0, 20) and (10, 70). What is its equation?
Correct answer: B
The slope is (70 − 20) ÷ (10 − 0) = 5 and the y-intercept is 20, so y = 5x + 20. y = 7x + 20 divides 70 by 10 without subtracting 20 first.
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Question 9
The points of a scatter plot lie very close to a line that rises from left to right. How is the association best described?
Correct answer: A
Points close to a line show a strong linear association, and a line rising to the right shows a positive one. Points spread far from the line would make it weak.
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Question 10
A line of best fit for a child’s height (cm) and age (years), built from data for ages 2 to 12, is y = 6x + 80. For age 40 it predicts 320 cm. What is the best conclusion?
Correct answer: B
The pattern only describes ages 2 to 12, and people stop growing in their late teens, so using the line far beyond the data gives a meaningless result. The arithmetic itself, 6 · 40 + 80 = 320, is correct.
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