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Answer to Problem 9:

1. Answers will vary.  A quick method for figuring tip would be to change the amount to 28 and find 10%, which is 2.8, and then 5%, which is 1.4.  Then, add the two to get $4.20 as the tip.  Another interesting method is to double the amount of tax that is charged, since the tax would be a little over 7%.
2. $27.83 x .15 = $4.17