Line of Best Fit Test – Grade 8 Math (Printable PDF)

Question 1 Use a model (2 points)

A model is y = 6x + 14.

1. Find y when x = 9. (1 point)
2. Find x when y = 104. (1 point)

Question 2 Read a trend line (3 points)

A drone’s distance from its pilot is graphed with a trend line.

1. Find the slope and the y-intercept of the line. (1 point)
2. Write the equation of the line. (1 point)
3. Use it to predict the distance at 14 minutes. (1 point)

Question 3 Meaning in context (3 points)

The water in a tank is modeled by y = −2.5x + 90, where x is minutes and y is gallons.

1. Interpret the slope. (1 point)
2. Interpret the y-intercept. (1 point)
3. When is the tank empty? (1 point)

Question 4 Choose the better line (3 points)

Two lines are drawn on the scatter plot.

1. Which line fits the data better? (1 point)
2. Give two reasons. (2 points)

Question 5 Equation from two points (3 points)

A trend line passes through (2, 19) and (8, 43).

1. Find the slope. (1 point)
2. Write the equation of the line. (1 point)
3. Predict y when x = 12. (1 point)

Question 6 Trusting a prediction (2 points)

A line fits a baby’s length from birth to 12 months. A student uses it to predict the length at 10 years and gets 140 inches. Explain what is wrong.

Question 7 Bracelet sales (2 points)

A fundraiser models its total bracelets sold as y = 1.5x + 8, where x is the number of days.

1. Interpret the slope. (1 point)
2. Predict the total after 20 days. (1 point)

Question 8 Candle burning (2 points)

A candle’s height is modeled by y = −0.75x + 18, where x is hours and y is inches. When is the candle 6 inches tall?