question: Tom's ship can travel at 10 miles per hour.  He is sailing from 1 to 4 PM.  He then travels back at a rate of 6 mph.  How long does it take him to get back?
answer: He was travelling at full speed for 4-1=<<4-1=3>>3 hours
  So he went 3*10=<<3*10=30>>30 miles
  That means it takes 30/6=<<30/6=5>>5 hours to get back
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