单选题
A certain truck traveling at 55 miles per hour gets 4．5 miles per gallon of diesel fuel consumed．Traveling at 60 miles per hour，the truck gets only 3．5 miles per gallon．On a 500-mile trip，if the truck used a total of 120 gallons of diesel fuel and traveled part of the trip at 55 miles per hour and the rest at 60 miles per hour，how many miles did it travel at 55 miles per hour?
【正确答案】
E
【答案解析】Let m be the number of miles the truck traveled at 55 miles per hour. It follows that 500 - m is the number of miles the truck traveled at 60 miles per hour. Then, m/4.5 is the number of gallons the truck used while traveling at 55 miles per hour and 
is the number of gallons the truck used while traveling at 60 miles per hour, so 
is the number of gallons the truck used while traveling at 60 miles per hour, so