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Let the speeds of boats ‘A’ and ‘B’ in still water be 8x km/hr and 9x km/hr Therefore, speed of the current = 0.25 × 8x = 2x km/hr According to the question, 8x + 2x = 640/8 Or, 10x = 80 Or, x = 8 Therefore, upstream speed of boat ‘B’ = 9x – 2x = 7x = 56 km/hr Downstream speed of boat ‘B’ = 9x + 2x = 11x = 88 km/hr Required time taken = (168/56) + (880/88) = 3 + 10 = 13 hours