A boat can go BA (upstream) in 84 minutes. The same boat when going downstream (AB) takes 9 minutes less than what it would take in still water. How much time will it take to reach from A to B (downstream)?

For a boat going upstream (against current) in 84 minutes, the distance is fixed. Downstream speed equals boat speed plus current speed, while upstream speed equals boat speed minus current speed. Still water time would be distance divided by boat speed alone. Using the relationship between these times and solving the equations shows downstream time is 63 minutes, which is option D.