let

v = the speed of the sailor in still water

c = the speed of the current

d = 8 km the distance in one direction

t1 = 40 min = 40/60 hr = 2/3 hr the time of the travel downstream

t2 = 1 hr the time of the travel upstream (return)

since speed = distance/time we have

the rate of the boat traveling downstream is: v + c

v + c = d/t1

v + c = 8/(2/3)

v + c = 12

the rate of the boat traveling upstream is: v - c

v - c = d/t2

v - c = 8/1

v - c = 8

by solving the system of equations

v + c = 12

v - c = 8

we find

v = 10 km/h

c = 2 km/h

click here to see the step by step solution of the system of equations

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the speed of the sailor in still water is 10 km/h.

the speed of the current is 2 km/h.