- A car completes a journey at an average speed of
40 m.p.h. At what speed must it travel on the
return journey if the average speed for the
complete journey (out and back) is 60 m.p.h.?
Answers
If the average speed for the complete journey (out and back) is 60 m.p.h. then the speed on the return journey must be 120 mph.
Step-by-step explanation:
Required formula:
Average speed = Total Distance / Total Time
⇒ Total time = total distance / average speed
Let the distance covered by the car for one way journey be “d” miles.
So, the total distance out and back will be d+d = 2d miles
Also, let the speed for the return journey of the car be “x” mph.
It is given that the average speed for the out journey is 40 mph and for the complete journey (out and back) is 60 mph
Therefore,
The time taken for the out journey = [d/40] hr
The time taken for the back journey = [d/x] hr
And,
The time taken for the complete journey = [2d/60] hr
Now, we can write the equation as,
[d/40] + [d/x] = [2d/60]
⇒ d[(x + 40) / (40x)] = [2d/60]
⇒ x + 40 = [2*40x] / 60
⇒ 60x + 2400 = 80x
⇒ 20x = 2400
⇒ x = 120 mph
Thus, the average speed of the car for the return journey must be 120 mph.
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