Correct Answer: 53. (1)

Solution: 53. (1) Let speed of current be $x$ kmph.

$\therefore$ Speed of boat in still water $=5 \times $ $\mathrm{kmph}$

$\therefore$ Its rate upstream $=6 \times $ $\mathrm{kmph}$

$\therefore$ Rate upstream $=6 x $ $\mathrm{kmph}$.

Rate upstream flow $=4 \times $ $\mathrm{kmph}$

According to the question,

$\frac{60}{6 x}+\frac{60}{4 x}=5$ $\Rightarrow \frac{10}{x}+\frac{15}{x}=5$

$\Rightarrow \frac{25}{x}=5 \Rightarrow x=\frac{25}{5}=5$ $ \mathrm{kmph}$.

$\therefore$ Speed of boat in still water

$=5 \times 5=25 $ $\mathrm{kmph}$ is incorrect. The unit $\mathrm{kmph}$ is not applicable to the result of a multiplication operation. The expression $5 \times 5=25$ is mathematically correct, but the unit $\mathrm{kmph}$ is not relevant unless the context specifies a speed measurement. If the intention is to express speed, the unit should be appropriately associated with the calculation.