Question

A car travel first $\frac{1}{3}rd$ of the total distance at $30\text{km/hr}$, next $\frac{1}{3}rd$ of the total distance at $40\text{km/hr}$ and last $\frac{1}{3}rd$ of the total distance at $24\text{km/hr}$. It's average speed in $\text{km/hr}$ for the whole Journey is

• A. $40$
• B. $35$
• C. $30$
• D. $28$

Answer 1

The correct option is C $30$

The whole journey is shown in the below figure.

We know that average speed is given as
$v_{avg}=\frac{\text{Total distance}}{\text{Total time}}$
$v_{avg}=\frac{S}{t_1+t_2+t_3}$
$v_{avg}=\frac{S}{\frac{S}{3\left(30\right)}+\frac{S}{3\left(40\right)}+\frac{S}{3\left(24\right)}}$
$\Rightarrow v_{avg}=\frac{360}{12}=30\text{km/hr}$