A boat covers a certain distance downstream in 1 hour, while it comes back in $$1\frac{1}{2}$$ hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?
a) 12 kmph
b) 13 kmph
c) 14 kmph
d) 15 kmph
Answer: d
Explanation: Let the speed of the boat in still water be x kmph
Speed downstream = (x + 3) kmph
Speed upstream = (x – 3) kmph
$$\eqalign{
& \left( {x + 3} \right) \times 1 = \left( {x – 3} \right) \times \frac{3}{2} \cr
& \Rightarrow 2x + 6 = 3x – 9 \cr
& x = 15\,\text{kmph} \cr} $$.
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