Subject: General Aptitude · Chapter: Boats and Streams · Exam: 2008 · Marks: · Difficulty:
A boat goes 30 km upstream and 44 km downstream in 10 hours. In 13 hours, it can go 40 km upstream and 55 km downstream. The speed of the boat in still water is
(a)3 km/hr
(b)4 km/hr
(c)8 km/hr
(d)None of these
Answer
Answer (as printed): C
Explanation
Let rate upstream $=x \mathrm{km} / \mathrm{hr}$ and rate downstream $=y \mathrm{km} / \mathrm{hr}$. Then, $\frac{30}{x}+\frac{44}{y}=10$ and $$\frac{40}{x}+\frac{55}{y}=13$$ Multiplying (i) by 4 and (ii) by 3 and subtracting, we get : $\frac{11}{y}=1$ or $y=11$. Putting $y=11$ in (i), we get : $x=5$. Hence, speed in still water $=\frac{1}{2}(11+5) \mathrm{km} / \mathrm{hr}=8 \mathrm{km} / \mathrm{hr}$.