Blackbox wrote:
A, B, K start from the same place and travel in the same direction at speeds of 30km/hr, 40km/hr, 60km/hr respectively. B starts two hours after A. If B and K overtake A at the same instant, how many hours after A did K start?
A) 3
B) 4.5
C) 4
d) 5.5
e) 5
Please include your explanation.
You can use relative speed concepts here:
B starts 2 hrs after A so A covers 30*2 = 60 km in that time while B is still at starting point.
B makes up 10 km of relative distance every hour because the difference in their speed is 10 km/hr so it takes 60/10 = 6 hrs to cover additional 60 kms and overtake A. So B overtakes A in a total of 2+6 = 8 hrs since A started.
A will be 30*8 = 240 kms away from the starting point at this time. This is also the point where K overtakes A.
K will cover this distance in 240/60 = 4 hrs. So K must have started 8-4 = 4 hrs after A.
Answer (C)
B overtakes A in 6 hrs, it took only 6 hours to do the distance of 240 km for B to overtake A. While A did 60 km, B was not moving. So how can you attribute additional 2 hours to B ?