A family travels from hither to yon one afternoon. On this day, the driver spends half the distance traveling at 77 km/hr and the remaining half at 50 km/hr. What is the average speed of the car during the trip?
To work the problem, I assumed the trip took 2 hrs, then found how much time was spent at each speed, then found a weighted average of 63.5 km/hr. This is incorrect.
Someone told me I can't assume the time it took to make the trip. Why not? Half is half, and the proportions will be the same..
How do I approach the problem?