Let the speed of man is u and that of current is v
Downstream speed will be u+v
Upstream speed will be u−v
He travels 30 km downstream in 5 hrs
So, downstream speed is 305=6 km/hr
Similarly upstream speed will be 185=3.6 km/hr
So, u+v=6
⇒u−v=3.6
Subtracting both the equations;
⇒2v=2.4
⇒v=1.2 km/hr