Subject: General Aptitude · Chapter: Boats and Streams · Exam: 2007 · Marks: · Difficulty:
A motorboat in still water travels at a speed of 36 km/hr. It goes 56 km upstream in 1 hour 45 minutes. The time taken by it to cover the same distance down the stream will be
(a)1 hour 24 minutes
(b)2 hour 21 minutes
(c)2 hour 25 minutes
(d)3 hour
Answer
Answer (as printed): A
Explanation
Speed upstream $$=\left(\frac{56}{1 \frac{3}{4}}\right) \mathrm{km} / \mathrm{hr}=\left(56 \times \frac{4}{7}\right) \mathrm{km} / \mathrm{hr}=32 \mathrm{km} / \mathrm{hr} .$$ Let speed downstream be $x \mathrm{km} / \mathrm{hr}$. Then, speed of boat in still water $=\frac{1}{2}(x+32) \mathrm{km} / \mathrm{hr}$. $$\therefore \quad \frac{1}{2}(x+32)=36 \Rightarrow x=40 .$$ Hence, required time $=\left(\frac{56}{40}\right) \mathrm{hrs}=1 \frac{2}{5} \mathrm{hrs}=1 \mathrm{hr} 24 \mathrm{min}$.