Question

A car travelled first third of a distance at 10 km/h, second third of the distance at 20 km/h and last third of the distance at 60 km/h. Mean speed of the car over entire distance is

• A. 30 km/h
• B. 20 km/h
• C. 18 km/h
• D. 54 km/h

Answer 1

The correct option is C 18 km/h

$v=\frac{d}{t}=\frac{d}{t_1+t_2+t_3}$
for first part of travelling;
$10=\frac{d}{3t_1}{t}_1=\frac{d}{3\times 10}$
for second part of travelling;
$t_2=\frac{d}{3\times 20}$
for third part of travelling;
$t_3=\frac{d}{3\times 60}$
mean speed $\left(v\right)$=$\frac{d}{\frac{d}{3}\left(\frac{1}{10}+\frac{1}{20}+\frac{1}{60}\right)}v=18km/h$
Hence, $v$= 18 km/h