Two boats on opposite banks of a river start moving towards each other. they first pass each other 1400 meters from one bank. they each continue to the opposite bank, immediately turn around and start back to the other bank. when they pass each other a second time, they are 600 meters from the other bank. we assume that each boat travels at a constant speed all along the journey. find the width of the river?

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