Problem Solving

Susan drove an average speed of 30 mph for the first 30 minutes of a trip. She did drove 60 mph for the next 30 minutes. If she made no stops, what was the average speed, in miles per hour, for the entire trip?


a. 35
b. 40
c. 45
d. 50
e. 55


Could someone please explain why my logic is flawed?

If the driver drives at an avg rate of x miles per hour and the driver only drives half of 1 hour shouldn't the distance driven be 1/2 of what it would be if a driver drove for one full hour?


first 30 minutes
She drove at avg speed for 30 mph, so if she drove for 1hr then she would driven 30 miles
but she only actually drove for 30 minutes or so would she only have driven 15 miles since she drove have of an hour

Second 30 minutes
She drove at avg speed for 60 mph, so if she drove for 1hr then she would driven 60 miles
but she only actually drove for 30 minutes or so wouldn't she only have driven 30 miles since she drove have of an hour

So Total distance for 1 hours=

15 miles (half of 30 miles since she drove for 30 min)+30 miles (half of 60 miles since she drove for 30 min) = 45 miles


So avg rate is D/Time or 45miles/1 hour