Answer :
assuming that the river doesn not move
if they kept a constand speed then they passes
the problem doesn't say how fast they were moving, but we can see that 1 boat is faster so
first pass
1400 meters from 1 bank
if they turn around they are 600 meters from the other bank
since they kept constant speed, they passed in the same place so
distance from bank1 to markx=1400
distance from bank2 to markx=600
distance from bank1 to bank2=600+1400
1400+600=total distance=2000 m
if they kept a constand speed then they passes
the problem doesn't say how fast they were moving, but we can see that 1 boat is faster so
first pass
1400 meters from 1 bank
if they turn around they are 600 meters from the other bank
since they kept constant speed, they passed in the same place so
distance from bank1 to markx=1400
distance from bank2 to markx=600
distance from bank1 to bank2=600+1400
1400+600=total distance=2000 m