Given:
Speed of current (y)= 6 km/hour
Distance = d km
Speed of boat in still water = x km/hour
Speed of the cruiser with the current= (x+6) km/hour
Speed of the cruiser against the current= (x-6) km/hour
[tex]\text{Time to travel with the stream=}\frac{d}{x+6}[/tex][tex]3=\frac{d}{x+6}[/tex][tex]3\mleft(x+6\mright)=d[/tex][tex]d=3x+18\ldots.\text{ (1)}[/tex][tex]\text{Time to travel }against\text{ the stream=}\frac{d}{x-6}[/tex][tex]7=\frac{d}{x-6}[/tex][tex]d=7x-42\ldots.\text{ (2)}[/tex]From equation (1) and (2)
[tex]7x-42=3x+18[/tex][tex]7x-3x=18+42[/tex][tex]4x=60[/tex][tex]x=15[/tex]Therefore the speed of the without a current is 15km/hour.
