Speed downstream = (13 + 4) km/hr = 17 km/hr.

Time taken to travel 68 km downstream =		68	hrs = 4 hrs.
		17

Man’s rate in still water = (15 – 2.5) km/hr = 12.5 km/hr.

Man’s rate against the current = (12.5 – 2.5) km/hr = 10 km/hr.

Let the man’s rate upstream be x kmph and that downstream be y kmph.

Then, distance covered upstream in 8 hrs 48 min = Distance covered downstream in 4 hrs.

		x x 8	4		= (y x 4)
			5

	44	x =4y
	5

y =	11	x.
	5

Required ratio =		y + x		:		y – x
		2				2

=		16x	x	1		:		6x	x	1
		5		2				5		2

= 8 : 3.

Let the speed of the stream be x km/hr. Then,

Speed downstream = (15 + x) km/hr,

Speed upstream = (15 – x) km/hr.

	30	+	30	= 4	1
	(15 + x)		(15 – x)		2

	900	=	9
	225 – x2		2

 9x² = 225

 x² = 25

 x = 5 km/hr.