question: James hires a horse-drawn carriage from 5 PM to 9 PM.  He gets 1 hour free.  The first paid hour is $15 and each hour after that is twice the cost.  How much did he pay?
answer: He got it for 9-5=<<9-5=4>>4 hours
  He pays for 4-1=<<4-1=3>>3 hours
  The first hour cost 1*15=$<<1*15=15>>15
  The other 3-1=2 hours are more expensive
  They cost 15*2=$<<15*2=30>>30 per hour
  So those 2 hours cost 2*30=$<<2*30=60>>60
  So he pays 60+15=$<<60+15=75>>75 per hours
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