Question

On a 120 km track, a train travels the first 30 km with a uniform speed of 30 km/h. How fast must the train travel the next 90 km so as to average 60 km/h for the entire trip?

• A. 65 km/h
• B. 90 km/h
• C. 120 km/h
• D. 100 km/h

Answer 1

The correct option is B 90 km/h

Average speed=Total distance Total time taken
$V_{av}=\frac{d}{t}ort=\frac{d}{V_{av}}$
For 120 km,
$t=\frac{120km}{60km/hr}=2hr$ ......(i)

For 1st 30 km,

$t_1=\frac{30km}{30km/hr}=1hr$

For remaining 90 km,
$t_2=t-t_1=2-1=1hr$
$speed=\frac{distance}{time}or{v}_2=\frac{d_2}{t_2}=\frac{90}{1}=90km/hr$