(d) Given speed of man `v_(m)=4km//h`
Speed of river `v_(r)=3km//h`
Width of the river `d=1km`
Time taken by the man to cross the river,
`t=("Width of the river")/("Speed of the man")=(1km)/(4km//h)=1/4 h = 1/4 xx60=15min`
Distance travelled along the river `=v_(r)xxt`
`=3xx1/4=3/4km=3000/4=750km`