Question

The speed of a boat in still water is 15Km/hr. It needs four more hours to travel 63 Km against the current river than it needs to travel down the river. Determine the speed of the current of the river.​

Answer 1

• The speed of the current of the river is 6 km/hr

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Let the speed of the current be x.

The speed with current is 15 + x and the speed against current is 15 - x.

The time difference to travel the distance of 63 km is 4 hours:

• 63/(15 - x) - 63/(15 + x) = 4
• 63(15 + x - 15 + x) = 4(15 + x)(15 - x)
• 63*(2x) = 4(225 - x²)
• 63x = 450 - 2x²
• 2x² + 63x - 450 = 0
• x = (-63 + √(63² + 2*4*450))/4 (negative root is discarded)
• x = (-63 + 87)/4
• x = 6