Time taken to cover first 50km at 100km/hr=‌1/2hr Time taken to cover second 50km at 50km/hr=1hr. Time taken to cover last 50km at 25km/hr=2 hr. When car 2 starts, car 1 has already covered 20km. So, time taken by car 1 to reach B after car 1 starts = total time − time required to travel first 20 km =3hr30min−12min=3hr18min Distance travelled by car 1=(50+50+45)=145km Distance from B=(150−145)km=5km Hence, 5 is the correct answer.