Math CentralQuandaries & Queries


Good afternoon!

An automobile travels 4 hours at an average speed of 42MPH. How much longer must the truck travel at an average speed of 55MPH so the average speed for the total trip will be 50MPH?

Thank you for your assistance!

Average speed is (total distance)/(total time).

For the 42 mph part of the trip he travels 4 × 42 = 168 miles in 4 hours.

Suppose he travels x hours at 55 mph. What is the distance travelled on this part of the trip?

Now write (total distance)/(total time) = 50 mph.

Solve for x.


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