A man rows a boat 18 kilometres in 4 hours downstream and returns upstream in 12 hours. The speed of the stream (in km per hour) isa.1.75b.1c.2d.1.5
Question
A man rows a boat 18 kilometres in 4 hours downstream and returns upstream in 12 hours. The speed of the stream (in km per hour) is:
a. 1.75
b. 1
c. 2
d. 1.5
Solution
The speed of a boat in still water and the speed of the stream can be calculated using the given distances and times.
When the man is rowing downstream, the speed of the boat and the speed of the stream add together. So, the speed of the boat downstream is 18km/4hrs = 4.5 km/hr.
When the man is rowing upstream, the speed of the stream subtracts from the speed of the boat. So, the speed of the boat upstream is 18km/12hrs = 1.5 km/hr.
The speed of the boat in still water is the average of the downstream and upstream speeds. So, (4.5 km/hr + 1.5 km/hr) / 2 = 3 km/hr.
The speed of the stream is the difference between the speed of the boat in still water and the speed of the boat upstream. So, 3 km/hr - 1.5 km/hr = 1.5 km/hr.
So, the speed of the stream is 1.5 km/hr. The answer is (d) 1.5.
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