On a trip from Fort Collins to Grand Lake, our Prius averaged 39 mpg for the first 63 miles (to the top of Trail Ridge Road in Rocky Mountain National Park). However, it averaged 51 mpg over the whole trip of 91 miles. How many miles per gallon did it get over the last 28 miles?

Solution to the Problem: The average mpg over the last part of the trip is 165.749 or 166 mpg. To find mpg, you must divide total miles by the total gallons. Let x = the MPG for the second part of the trip. Set up the following table to help you solve it:     MPG     Gallons     Miles     1st Part   39     21 / 13     63   2nd Part   x   28 / x   28   Whole Trip     51   91 / 51   91 The number of gallons used on the first part of the trip plus the number of gallons used on the second part must equal the total number of gallons used for the trip, so this gives us our equation: 21 / 13 + 28 / x = 91 / 51 1.65153 + 28 / x = 1.784 Then 28 / x = .1689291105, so x = 165.74999 or 166 mpg.

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