world problem

Question

The current in a stream moves at a speed of 9 km divided by h km/h. A boat travels 28 km upstream and 28 km downstream in a total time of 20 hr. What is the speed of the boat in still water?

Answer 1

d = r · t

d / r = t

28_up / (r - 9) = t_up

28_down / (r + 9) = t_down

t_up + t_down = 28

28 / (r - 9) + 28 / (r + 9 = 28

 

Can you solve for r and answer?