conversations.from: human
conversations.value: John rents a car to visit his family.  It cost $150 to rent the car.  He also had to buy 8 gallons of gas to fill it up and gas is $3.50 per gallon.  The final expense is $.50 per mile.  If he drove 320 miles how much did it cost?
  What is the solution?
conversations.from: assistant
conversations.value: The gas cost 8*3.5=$28
  The mileage expenses cost 320*.5=$160
  So in total he paid 150+28+160=$338