`t_(1)=(AB)/(v_(b)+v_(R)) . T_(2)=(AB)/(v_(b)-v_(R))` or `v_(b)+v_(R)=(AB)/(t_(1))`
and `v_(b)-v_(R)=(AB)/(t_(2))rArr 2v_(b)=(AB)/(t_(1)+(AB)/(t_(2)))=AB((t_(1)+t_(2))/(t_(1)t_(2)))`
or `((2t_(1)t_(2))/(t_(1)+t_(2)))=(AB)/(v_(b))`= time taken by the boat to cover AB