You first need to calculate how many miles the car traveled:
(60 miles/hour) x (1 hour/60 min) x (1 min/60 sec) x 100.0 sec = distance traveled (in miles)
Then to get miles per gallon, divide the distance traveled (calculated above) by amount of gas consumed (0.1 gal). That's it.
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Won't you help a poor little puppy?