A tire manufacturer is interested in testing the fuel economy for two different tread patterns. Tires of each tread type are driven for 1000 miles on each of 18 different cars. The mileages, in mpg, are presented in the following table.

Car |
Tread A |
Tread B |

1 |
24.1 |
20.3 |

2 |
22.3 |
19.7 |

3 |
24.5 |
22.5 |

4 |
26.1 |
23.2 |

5 |
22.6 |
20.4 |

6 |
23.3 |
23.5 |

7 |
22.4 |
21.9 |

8 |
19.9 |
18.6 |

9 |
27.1 |
25.8 |

Car |
Tread A |
Tread B |

10 |
23.5 |
21.4 |

11 |
25.4 |
20.6 |

12 |
24.9 |
23.4 |

13 |
23.7 |
20.3 |

14 |
23.9 |
22.5 |

15 |
24.6 |
23.5 |

16 |
26.4 |
24.5 |

17 |
21.5 |
22.4 |

18 |
24.6 |
24.9 |

a. Find a 99% confidence interval for the mean difference in fuel economy.

b. A confidence interval based on the data in the table has width ±0.5 mpg. Is the level of this confidence interval closest to 80%, 90%, or 95%?

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