find the rate of the man in still water

let the velocity of man in still water be x

when the man goes upstream his resultant speed is (x-2)

time taken by him to cover 30 km upstream = 30/(x-2)

when the man goes downstream his resultant speed is (x+2)

time taken by him to cover 30 km upstream = 30/(x+2)

total time = 30/(x-2)+30/(x+2)=8

so x=8