Subject: General Aptitude · Chapter: Boats and Streams · Exam: · Marks: · Difficulty:
A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph , the speed of the stream is :
(a)2 mph
(b)2.5 mph
(c)3 mph
(d)4 mph
Answer
Answer (as printed): A
Explanation
Let the speed of the stream be $x \mathrm{mph}$. Then, Speed downstream $=(10+x) \mathrm{mph}$, Speed upstream $=(10-x) \mathrm{mph}$. $$\begin{aligned} \therefore \quad & \frac{36}{(10-x)}-\frac{36}{(10+x)}=\frac{90}{60} \Leftrightarrow 72 x \times 60=90\left(100-x^{2}\right) \\ & \Leftrightarrow x^{2}+48 x+100=0 \\ & \Leftrightarrow(x+50)(x-2)=0 \Leftrightarrow x=2 \mathrm{mph} . \end{aligned}$$
Explanation as extracted from the printed page; notation may be imperfect.