Answer: 2.4km
In this case, Murphy is rowing to a place and come back so he was going 2x of the place distance. That means when he goes to the place the current velocity is helping him(5+1 kmph) and when he comes back the current velocity is slowing him(5-1kmph).
To calculate the place distance, the calculation would be:
X/6kmph + X/4kmph= 1 hour
X/6kmph + 1.5X/6kmph= 1 hour
2.5X/6kmph= 1hour
2.5X= 1 hour x 6km/ hour
X= 6km/2.5
X= 2.4km