86. X, Y, Z start from place A at 5 kmph, 8 kmph and 11 kmph respectively. If Y starts 2 hours after
X, how many hours after Y should Z start, so that both Y and Z overtake X at the same time?
(a) 2 hours (b) (5/16) hours (C) (16/11) hours (d) (10/11) hours (e) (11/5) hours
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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)
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