arithmetic - a boat covers a certain distance downstream in 1 hour while it comes back in 1 and half hours if the speed of the stream be 3 kmph what is the speed of the boat in still water - skillgun

Let the speed of the boat in still water be x kmph. Then,
Speed downstream = (x + 3) kmph,
Speed upstream = (x - 3) kmph.
=> (x + 3) x 1 = (x - 3) x 3/2
=> 2x + 6 = 3x - 9
=> x = 15 kmph.