old_contents: from itertools import groupby
  ​
  def total_cost(calls):
      total = 0
      for k,g in groupby(calls, lambda x:x[:10]):
          minutes = sum(-(-int(l[20:])//60) for l in g)
          total += (minutes, 2*minutes-100)[minutes>100]
      return total
  
new_contents: from itertools import groupby
  
  def total_cost(calls):
      total = 0
      for k,g in groupby(calls, lambda x:x[:10]):
          minutes = sum(-(-int(l[20:])//60) for l in g)
          total += (minutes, 2*minutes-100)[minutes>100]
      return total
  
subject: Update calls-home.py