Question 736018
let speed of current be c kph

upstream speed = 10-c
upstream distance = 12 

down stream speed = 10+c
down stream distance = 28 km

upstream time + downstream time = 4

t=d/r

12/(10-c) + 28/(10+c)= 4

multiply the equation by (10+c)(10-c)


12(10+c)+28(10-c)=4(10+c)(10-c)

120+12c+280-28c=4(100-c^2)

400-16c=400-4c^2

re arrange
4c^2=16c

/4c
c=4


current speed = 4 kph