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Let's take the scheduled time taken by the bus to be t
From the first statement (bus travelling at 60 kmph), we can get the total distance travelled by bus to 60(t + 3.5)
The second scenario gives us that the bus covered two-thirds of the distance in one-third of the time, meaning that the remaining one-third distance was covered in two-thirds of the time, giving us the relation (1/3)st covered in (2/3)t giving the speed to be  s/2 which is given as 40 km/h, thereby giving the usual speed of the bus to be 80 km/hr
Now the first relation we get 60(t + 3.5) = 80t
Giving us t = 10.5 hours
Thus, the bus usually takes 10.5 hours on its journey.
Staring at 9:00, it will complete the journey at 7:30 pm
Therefore, option b is the correct answer.