8.4 A random sample of 10 pairs of identical houses was chosen in a large Midwestern city, and a passive solar heating system was installed in one house from each pair. The total fuel bills (in dollars) for three winter months for these homes were then determined, as shown in the accompanying table. Assuming normal population distributions, find a 90% confidence interval for the difference between the two population means.

Pair Without Passive Solar With Passive Solar Pair Without Passive Solar With Passive Solar 1 485 452 6 386 380 2 423 386 7 426 395 3 515 502 8 473 411 4 425 376 9 454 415 5 653 605 10 496 441