Answer:
The correct option is;
Dennis must have stopped for an hour in the middle of this trip
Step-by-step explanation:
The given parameters are;
The distance Dennis covered in 5 hours = 500 km
Dennis's average speed in the first two hours = 150 km/h
Dennis's average speed in the last two hours = 100 km/h
Therefore;
Dennis traveled 150 km/h × 2 h = 300 km in the first two hours
Dennis traveled 100 km/h × 2 h = 200 km in the last two hours
Which gives, Dennis traveled 300 km + 200 km in 2h + 2h = 4h
Therefore, Dennis traveled 500 km in 4 hours and Dennis must have stopped for an hour in the middle of this trip.