Correct option is A
Let the speed of boat in still water and speed of stream is X and Y km/hr respectively.
(X – Y) – Y = 12
X – Y – Y = 12
X -2Y = 12
X = 12 + 2Y….(I)
And
18 = X + Y
18 = 12 + 2Y + Y (From I)
6 = 3Y
2 = Y
X = 12 + 2(2)
X = 16
Required distance = 5×(16-2)=70 km