Sheila interested in how many gallons of gasoline her neighbors use per week. She collected the following data from all of her neighbors.

7used between 1 and 20 gallons, 10used between 21 and 40 gallons, 4used between 41 and 60gallons, and 5used between 61and 80gallons

Calculate the mean gallons of gasoline used per week. Keep all intermediate steps to four decimals then enter your answer to two decimals.