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2014-09-16T18:44:31+02:00
A= 6/7-3/4*8/15
  =  6/7-3*8/4*15
  = 6/7-24/60
  = 6*60/7/60.24*7/60*7
  = 360/420-168/420
  =192/420
  = 16/35