Correct Option: C

Let the speed of the motorboat in still water be Z km per hr.
Downstream speed= (Z + 3) km per hr.
Upstream speed = (Z – 3) km per hr.
Total journey time

= 30 minutes =	30	hr. =	1	hour
	60		2

We can write,

2	+	2	=	1
Z − 3		Z + 3		2

or,   2		(Z + 3) + (Z − 3)			=	1
		(Z − 3) + (Z − 3)				2

or,	2Z	=	1
	Z2 − 9		4

or,   Z² – 9 = 8Z
or,   Z² – 8Z – 9 = 0
or,   Z² + Z – 9Z – 9 = 0
or,   Z(Z + 1) – 9 (Z + 1) = 0
or,   (Z + 1) (Z – 9) = 0
∴  Z = – 1 or 9.
Since speed can’t be negative
Therefore, the speed of the motor-boat in still water = 9 km per hr.