A man can row three-quarters of a kilometre against the stream in 11$ \dfrac{1}{4} $ minutes and down the stream in 7$ \dfrac{1}{2} $ minutes. The speed in km/hr of the man in still water is:
2
3
4
5
Explanation:
We can write three-quarters of a kilometre as 750 metres,
and 11$\dfrac{1}{4}$ minutes as 675 seconds.
| Rate upstream =$ \left(\dfrac{750}{675} \right) $m/sec=$ \dfrac{10}{9} $m/sec. |
| Rate downstream =$ \left(\dfrac{750}{450} \right) $m/sec=$ \dfrac{5}{3} $m/sec. |
| $\therefore$ Rate in still water =$ \dfrac{1}{2} \left(\dfrac{10}{9} +\dfrac{5}{3} \right) $m/sec |
| =$ \dfrac{25}{18} $m/sec |
| =$ \left(\dfrac{25}{18} \times\dfrac{18}{5} \right) $km/hr |
= 5 km/hr.