Relative motion

1/6 h
8/6 km

for this u just need to know that cos of 90 degree is 0 or the effect of the perpendicular vector on another vector is 0 that is the man would reach the other bank of the river only due to his velocity and not due to the rivers velocity.so the man must start perpendicular to the river flow. Now the time taken by him will be the least 1 and that will be equal to distance divided by his speed. For the second part the distance traveled along d velocity of the river would be = to the product of time taken to reach the other bank and the velocity of the river which pushes him along that direction.