Question

A bus going from Kota to Jaipur just passed the 100 km, 160 km, and 220km points at 10:30 am, 11:30 am and 1:30 pm. The average speed of bus during 11:30 am to 1:30 pm is
60 km/hr
30 km/hr
(C) 40 km/h
(D) 120 km/hr

Answer 1

Hint: Get the distance covered between the time 11:30 am and 1:30 pm. Here, the time interval is 2 hr. Now, put the values of distance and time interval in the formula, average speed = \[\dfrac{\text{Total}\,\text{distance covered}}{\text{Total time}\,\text{interval}}\] . Now, solve it further and get the average speed.

Complete step-by-step answer:

At 11:30 am, the bus passed the 160 km point.
At 1:30 pm, the bus passed 220 km.
The net distance covered between the time 11: 30 am and 1:30 pm is the distance BC as shown in the figure.
So,the net distance covered between the time 11:30 am and 1:30 pm = 220-160 km = 60 km.
We can write the time 1:30 pm as 13:30 pm.
Time interval = 13:30-11:30 = 2 hr.
We know the formula of average speed = \[\dfrac{\text{Total}\,\text{distance covered}}{\text{Total time}\,\text{interval}}\] .
From the equation () and equation (), we have the total distance covered and the total time interval.
Now, putting it in the formula, we get
Average Speed = \[\dfrac{\text{Total}\,\text{distance covered}}{\text{Total time}\,\text{interval}}=\dfrac{60km}{2hr}=30\,km/hr\] .
So, the average speed of the bus is 30 km/hr.
Hence, the correct option is (C).

Note: In this question, one may think that the distance covered between the time 11:30 am and 1:30 pm is 160 km + 220 km = 380 km. If we do so, then our average speed will be,
Average Speed = \[\dfrac{\text{Total}\,\text{distance covered}}{\text{Total time}\,\text{interval}}=\dfrac{380km}{2hr}=190\,km/hr\] .
We get the average speed of the bus = 30 km/hr and it is wrong. The distance (160 km + 220 km) is not covered between the time 11:30 am and 1:30 pm. They are only the km points, which the bus passes.