A start walking from a place at a uniform speed of 6 kmph in a particular direction. After half an hour, B starts from the same place and walks in the same direction as A at a uniform speed and overtakes A after 1 hour 48 minutes. Find the speed of B. Choose the correct choice from the following choices.
A) 4.7 kmph B) 6.6 kmph C) 4 kmph D) 7 kmph E) 5.3 kmph

Rationale: "distance covered by a in 30 min = 1 km b covers extra 1 km in 1 hour 48 minutes ( 9 / 5 hr ) i . e . relative speed of b over a = 1 / ( 9 / 5 ) = 5 / 9 so the speed of b = speed of a + 5 / 9 = 6 + 5 / 9 = 6.55 answer b"
Answer: B