Miscellaneous Question 102
- What is the average speed for total journey, when a boat rows at a speed of $12 $ $km / h$ in still water to an upstream point and back to the starting point in a canal which flows at $3$ $ km / h$ ?
(1) $11 \frac{1}{4} $ $km / h$
(2) $9 \frac{1}{3} $ $km / h$
(3) $10 \frac{3}{4}$ $ km / h$
(4) $12 \frac{1}{5} $ $km / h$
(5) $16 \frac{1}{2}$ $ km / h$
(IBPS RRBs Officer CWE Prelim Exam, 07.08.2021)
Show Answer
Correct Answer: 102. (1)
Solution: 102. (1) Speed downstream $=(12+3)=15 $ $\mathrm{km} / \mathrm{h}=x$
Speed upstream $=(12-3)$
$=9$ $ \mathrm{km} / \mathrm{h}=y$
$\therefore$ Required average speed
$=\left(\frac{2 x y}{x+y}\right)$ $ \mathrm{kmph}$ as up and down distances are same.
$=\left(\frac{2 \times 15 \times 9}{15+9}\right) $ $\mathrm{kmph}$
$=\left(\frac{2 \times 15 \times 9}{24}\right)$ $ \mathrm{kmph}$
$=\frac{45}{4} $ $\mathrm{kmph}$
$=11 \frac{1}{4}$ $ \mathrm{kmph}$
BETA
