Speed = distance/time

Let the speed of sailor in still water be ‘a’ and speed of the current be ‘b’.

Relative speed of sailor going upstream = a – b

Relative speed of sailor going downstream = a + b

Given, sailor goes 8 km downstream in 40 minutes and returns in 1 hour.

⇒ a + b = 12 ----- (1)

Also,

a – b = 8/1 = 8 ------ (2)

Adding (1) and (2).

⇒ 2a = 20

⇒ a = 10km/hr

Thus, b = 12 – 10 = 2 km/hr