In a test of the effectiveness of a device that is supposed to increase gasoline mileage in automobiles, 12 cars were run, in random order, over a prescribed course both with and without the device in random order. The mileages (mpg) are given in Table 5.16. Is there evidence that the device is effective? 

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Table 5.16 Data for Exercise 8. Car No. 1 Without Device With Device 21.0 20.6 12345678 30.0 29.9 29.8 30.7 27.3 26.5 27.7 26.7 33.1 32.8 18.8 21.7 26.2 28.2 9 28.0 28.9 10 18.9 19.9 =2 11 29.3 32.4 12 21.0 22.0