River speed =5m/min
And Man speed =10m/min
Let the width of river =xmtr
He travels due north then his rtesultant velocity acts 30 degree north of east and the distance to be covered to reach the other bank increase to xcos30o
And resultant velociity =11.2m/min
So time taken x9.7mins
If the travels due 30 degree north of east then his resultant velocity (Due to velocity of downstream of water ) acts due north.
The distance be covered to reach the other bank will be x mtrs.
And the resultant velocity is 8.7mtr/min.
So time taken is x8.7mins.
Then, 30 degree east of northSo at least time he should travel due to north.