Question

An automobile travels from city A to city B and returns to city A by the same route. The speed of the vehicle during the onward and return journeys were constant at 60 km/h and 90 km/h, respectively. What is the average speed in km/h for the entire journey?

• A.
  72
• B.
  73
• C.
  74
• D.
  75

Answer 1

The correct option is A

72

$Totaldistance=x$

$\left(Onwardjourney\right)S_1=60km/h$
$\left(Returnjourney\right)S_2=90km/h$

$\text{Average Speed}=\frac{\text{Total distance}}{\text{Total time}}$

$=\frac{x+x}{\frac{x}{60}+\frac{x}{90}}=\frac{2x}{x\left[\frac{3+2}{90\times 2}\right]}$

$=\frac{2\times 90\times 2}{5}=72km/h$