A boat takes a total time of twelve hours to travel 105 kms upstream and the same distance downstream. The speed of the boat in still water…

Solution
Let speed of current = $x$ km/hr
=> Speed of boat in still water = $6x$ km/hr
Acc. to ques, => $\frac{105}{7x} + \frac{105}{5x} = 12$
=> $\frac{15}{x} + \frac{21}{x} = 12$
=> $\frac{36}{x} = 12$
=> $x = \frac{36}{12} = 3$
$\therefore$ Speed of boat in still water = $6 \times 3 = 18$ km/hr