Question : The speed of a boat in still water is thrice the speed of the stream. If the boat takes 15.5 sec to go to a certain place downstream, then find the additional time required to cover the same distance travelling upstream.
Option 1: 15.5 sec
Option 2: 29 sec
Option 3: 31 sec
Option 4: 35 sec
Correct Answer: 15.5 sec
Solution :
Let the speed of the stream as v and the speed of the boat in still water as 3v.
When the boat is moving downstream (i.e., with the current), its effective speed is the sum of its own speed and the speed of the stream, which is 3v + v = 4v
If the boat takes 15.5 seconds to cover a certain distance downstream, the distance as d = 4v × 15.5
When the boat is moving upstream (i.e., against the current), its effective speed is the difference between its own speed and the speed of the stream, which is 3v – v = 2v
The time it takes to cover the same distance upstream, t = $\frac{\text{d}}{\text{2v}}$ = $\frac{(\text{4v × 15.5})}{ (\text{2v})}$ = 31 sec
$\therefore$ The additional time required to cover the same distance travelling upstream is 31 – 15.5 = 15.5 sec
Hence, the correct answer is 15.5 sec.
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