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A car drives from A to B, half the time at 55 km/h and the other half at 90 km/h. On the way back, the car travels half the distance at 55km/h and the other half at 90km/h. What is the average speed from B to A?

Thanks in advance!

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- #1

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A car drives from A to B, half the time at 55 km/h and the other half at 90 km/h. On the way back, the car travels half the distance at 55km/h and the other half at 90km/h. What is the average speed from B to A?

Thanks in advance!

- #2

HallsofIvy

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The problem doesn't tell us what the distance between A and B is so let's call it "D" km.

Notice that the problem only asks for the average speed driving back from B to A. All the first part, about how the person got from A to B is irrelevant.

The person drives back from B to A (presumably the same distance D!) going half the distance at 55 mph. If we let T

He then drives half the distance at 90 km/h. If we let T

To find "average speed", we need to divide the distance by the total time so we need to find total time:

The total time is T

The average speed is D/((29/1980)D)= 1980/29= 68.27 km/h.

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