Bernardo drives to work at an average speed of 50 miles per hour and returns along the same route at an average speed of 25 miles per hour. if his total travel time is 3 hours, what is the total number of miles in the round-trip?

Bernardo travels the same distance at 25mph as he does at 50mph. However, since 25mph is only half of 50 mph, he must travel twice as long at 25mph. If you call the time he traveled 50mph "t", then 

t+2t=3 
3t=3 
t=1 

This means he traveled 1 hour at 50mph. In this time, he traveled 50 miles. He traveled the same distance at 25mph, so his total distance was 

50miles+50miles=100miles 

so the round trip was 100 miles.