A streamer goes downstream from one port to another in 4 hours. It covers the same distance upstream in 6 hours. If the speed of the current in 1.5 km/ hr, then the distance between the two ports is?

Solution: Let the steamer in still water is x km/hr

Now,  speed of  steamer in downstream is (x + 1.5) km/hr

So,  distance $=(x+1.5)\times 4$

$(4x+6)km/hr$

Speed of  steamer in upstream is $=(x-1.5)km/hr$

$So,\text{\hspace{0.17em}\hspace{0.17em}distance}=(x-1.5)\times 6$

$=(6x-9)km/hr$

Speed of  boat $=4x+6=6x-9$

$2x=15$

$x=7.5km/hr$

$\therefore \text{\hspace{0.17em}\hspace{0.17em}Distance}=(4x+6)$

$=(4\times 7.5+6)$

$=(30+6)=36km/hr$