Question 839076
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Hi
The speed of a stream is 6mph
If a boat travels 54 miles downstream in the same time that it takes to travel 27 miles upstream, 
what is the speed {{{highlight_green(x)}}} of the boat in still water?
D = rt  0r   t = D/r
Question states***
 {{{ 54/(x + 6mph) = 27/(x - 6mph)}}}  |Cross Multiply to solve for x

A cruise boat travels 12 miles downstream in 4 hours and returns to its starting point upstream in 6 hours. 
 Find the speed of the stream
  (r+c)4hrs = 12mi
  ((r-c)6hrs = 12mi
 4r + 4c = 12
 6r - 6c = 12
Solving for r by Elimination of c:
 24r + 24c = 72
 24r - 24c = 48
    48r = 120
      r = 2.5mph, speed of boat in still water   (current =  .5mph)