← Back to the Line Of Best Fit worksheet
Last updated: October 22, 2026 · Written by the Math Made Easy math teachers
Standards: 8.SP.A.2, 8.SP.A.3 · Level: Grade 8
Problem 2 Plant model
The slope 2.5 means the plant grows about 2.5 cm per week.
The intercept 12 is the height at x = 0, the first measurement.
Slope: 2.5 cm per week; intercept: 12 cm at the start
Problem 3 Which line fits?
Line A is y = 2x + 5. It gives 7, 9, 11, 13, 15, 17, 19, 21, close to the data with points on both sides.
Line B is y = x + 9. It gives 10 to 17, and the points on the right sit far above it.
Line A fits better
Problem 4 Equation from two points
Slope: (23 − 11) ÷ (6 − 2) = 12 ÷ 4 = 3.
Use (2, 11): 11 = 3 · 2 + b, so b = 5.
y = 3x + 5
Problem 5 Draining pool
y = −4 · 7 + 90 = −28 + 90.
62 gallons
Problem 6 Count the points
The line gives 4.5, 7, 9.5, 12, 14.5, 17, 19.5, 22 for x = 1 to 8.
Above the line: (1, 5), (3, 11), (5, 15). Below: (4, 11), (7, 18). On the line: (2, 7), (6, 17), (8, 22).
3 above, 2 below, 3 on the line: a good fit
Problem 7 Used car model
The slope −1.7 means the car loses about 1.7 thousand dollars of value each year.
The intercept 20.6 is the value at age 0, when the car is new.
Loses about $1,700 per year; new value about $20,600
Problem 8 Equation from a graph
Slope: (23 − 3) ÷ (10 − 0) = 2. The line meets the vertical axis at 3.
y = 2x + 3
Problem 9 Predict from the model
y = 2 · 7.5 + 3 = 15 + 3.
18
Problem 10 Reach a target
2x + 3 = 21, so 2x = 18.
x = 9
Problem 11 Prediction error
Predicted: 5 · 4 + 51 = 71.
Actual minus predicted: 70 − 71 = −1.
The actual score is 1 point below the prediction
Problem 12 Is the prediction safe?
3.5 hours: 5 · 3.5 + 51 = 68.5, inside the data range.
20 hours: 5 · 20 + 51 = 151, outside the range and above the maximum score of 100.
68.5 is reasonable; 151 is not
Problem 13 Handle the outlier
The line should follow the overall trend of the ten points. One point that is far from the pattern should not pull the line toward it.
First check whether it is a mistake or an unusual case; then draw the line for the main trend.
No, the line should follow the ten points, not the outlier
Problem 14 Equation from a table
Slope: (17 − 9) ÷ (3 − 1) = 4. Then 9 = 4 · 1 + b, so b = 5.
Equation: y = 4x + 5. For x = 12: 4 · 12 + 5 = 53.
y = 4x + 5; at x = 12, y = 53
Problem 15 Bike rental
Slope 14: each hour costs $14. Intercept 30: a flat $30 fee before any hours.
y = 14 · 5.5 + 30 = 77 + 30 = 107.
$14 per hour, $30 flat fee; 5.5 hours cost $107
Problem 16 Battery life
After 5.5 hours: −12 · 5.5 + 100 = −66 + 100 = 34.
For 20%: −12x + 100 = 20, so −12x = −80 and x = 80 ÷ 12 ≈ 6.7.
34% after 5.5 hours; 20% at about 6.7 hours
Problem 17 Candle burning
After 10 hours: −0.8 · 10 + 24 = 16.
Burns out when y = 0: −0.8x + 24 = 0, so x = 24 ÷ 0.8 = 30.
16 inches after 10 hours; burns out after 30 hours
Problem 18 Savings plan
12x + 85 = 300, so 12x = 215 and x = 215 ÷ 12 ≈ 17.9.
At 17 weeks: 12 · 17 + 85 = 289, which is under $300. At 18 weeks: 12 · 18 + 85 = 301.
Week 18
Problem 19 Compare two models
Model A gives 13.8, 24, 34.2. Differences: 0.2, 1.0, 0.2. Total 1.4.
Model B gives 14, 23, 32. Differences: 0, 0, 2. Total 2.
Model A is closer overall (total difference 1.4 versus 2)
Problem 20 Babysitting earnings
Slope: (62 − 10) ÷ 8 = 6.5. Intercept: 10. Equation: y = 6.5x + 10.
The slope means about $6.50 earned per hour; the intercept means a $10 starting amount, such as a bonus.
For 10 hours: 6.5 · 10 + 10 = 75.
y = 6.5x + 10; $75 for 10 hours