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world problem

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?
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1 Answer

d = r ยท t
d / r = t
28up / (r - 9) = tup
28down / (r + 9) = tdown
tup + tdown = 28
28 / (r - 9) + 28 / (r + 9 = 28
 
Can you solve for r and answer?