Resultant Force Help: Converting m/s to mph

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johnnyapplese
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I was relearning some physics and I came across this problem.

Kyle and Paul, two NHL players, are playing hockey against each other and Kyle checks Paul, on center ice. They hang on to each other so the collision is inelastic. Assume Kyle weighs 90kg and Paul weighs 80kg. Kyle was skating at 5m/s and Paul at 8m/s. They collide head on.

I figured out the answer was 1.11m/s easily enough, but then i wondered if I could convert the m/s into mph and get the same answer but I couldn't get the same answer. My question is why can't you simply convert 8m/s and 5m/s into mph and then later convert it back to get the right answer. I don't see what's wrong with it.
 
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You can convert to mph and you should get the same answer. You might be making an error in conversion.