The speed of motor boat is that of the current of water as 91:9 The boat goes along with current in 10 hours 15 minutes. It will come back in ?

Let the speed of  the motor boat be 91x km/hr and that  of  the current water be 9x km/hr

Speed down stream = (91x + 9x) = 100x km/hr

Up stream = (91x – 9x) = 82x km/hr

Time taken to down stream = 10 hour 15 min

$=10\frac{15}{60}=\frac{615}{60}=\frac{41}{4}hrs$

Distance covered down stream $=(100x\times \frac{41}{4})km$

Distance covered up stream $=(82x\times \frac{41}{4})km$

$\therefore$ time taken to travel up stream

$=(100x\times \frac{41}{4}\times \frac{1}{82x})=\frac{25}{2}hrs$

$=12.5hrs$

$=12hrs30\min$