A train travels at an average speed of 20 mph through urban areas, 50 mph
through suburban areas, and 75 mph through rural areas. If a trip consists of
traveling half an hour through urban areas, 3-1/2 hours through suburban areas,
and 3 hours through rural areas, what is the train’s average speed for the entire
trip?
a. 50 mph
b. 53-2/7 mph
c. 54-3/7 mph
d. 58-4/7 mph
e. 59-2/7 mph

can any one tell how to solve it?