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the monthly cost in dollars of a long-distance phone plan is a linear function
of the total calling time in minutes.
The monthly cost for 43 minutes is $13.42 and the monthly cost for 91 minutes is $19.66.
What is the monthly cost for 55 minutes?
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The price per minute is m = = 0.13 dollars per minute.
The montly cost for 55 minutes is 13.42 + m*(55-43) = 13.42 + 0.13*(55-43) = 14.98 dollars. ANSWER
Solved.