Distance between two stations A and B is 778 km. A train covers the journey from A to B at 84 km per hour and returns back to A with a uniform speed of 56km per hour. Find the average speed of the train during the whole journey?
69.0 km /hr
69.2 km /hr
67.2 km /hr
67.0 km /hr
Explanation:
If a car covers a certain distance at $x$ kmph and an equal distance at $y$ kmph. Then,
average speed of the whole journey = $\dfrac{2xy}{x+y}$ kmph.
By using the same formula, we can find out the average speed quickly.
Average speed
$=\dfrac{2 \times 84 \times 56}{84 + 56}=\dfrac{2 \times 84 \times 56}{140}$
$=\dfrac{2 \times 21 \times 56}{35}=\dfrac{2 \times 3 \times 56}{5}$
$=\dfrac{336}{5}=67.2$