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7-103.

Marissa’s older sister was discussing purchasing a car with her summer job earnings.  Marissa collected data from her friends at her job. Homework Help ✎

Refer to problem 7-92 for help.

1. Marissa would like to know the typical age of her friends’ cars.  What kind of graphical display should she use?

A box plot shows center and spread of one-variable numerical data nicely.

1. Marissa wants to convince her dad that newer cars are more fuel-efficient.  What kind of graph(s) should Marissa make to convince her dad?

Marissa wants to show her dad an association between age and miles per gallon.

1. Make a scatterplot of the data.

Use age as the $x$-axis and mpg as the $y$-axis.

1. Fully describe the association.

Weak, negative, linear association.

1. Draw a line of best fit on the data.  Find the equation of the line of best fit.

Remember that when drawing a line of best fit, about half the points should be above the line and about half should be below.

2. Use the equation to predict what the correct mileage for a $7$-year-old car should be.

• You should get around $31$ mpg.

1. Interpret the slope and $y$-intercept in this situation.

A slope of $-0.7$ means that for each additional year old a car is, it is expected to go $0.7$ less miles per gallon.
What does the $y$-intercept say about a new car?

 Age of Car (years) Fuel Efficiency (mpg) 9 34 2 30 5 30 1 35 4 34 10 28 9 27 3 40 8 29