Question : Speed of stream is 4 km/hr and the speed of boat is 11 km/hr. In how much time will the boat cover a distance of 21 km upstream and 45 km downstream?
Option 1: 6 hours
Option 2: 3 hours
Option 3: 4 hours
Option 4: 7 hours
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Correct Answer: 6 hours
Solution :
The speed of the stream is 4 km/hr.
The speed of the boat is 11 km/hr.
Upstream Speed = Speed of boat –Speed of stream
= 11 – 4 = 7 km/h
Upstream distance = 21 km
Time = $\frac{21}{7}$ = 3 hrs
Downstream Speed = Speed of boat + Speed of stream
= 11 + 4 = 15 km/h
Downstream distance = 45 km
Time = $\frac{45}{15}$ = 3 hrs
Total time = 3 + 3 = 6 hrs
Hence, the correct answer is 6 hours.
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