def add (*numbers)
  numbers.inject(0) { |sum, number| sum + number }
end

def subtract(*numbers)
   numbers.inject() { |sum, number| sum - number }
end

def calculate(*array)
  calculation =  array.last
  if calculation.is_a?(Hash)
    numbers = array[0..-2]
  else
    calculation [:add] =  true
    numbers = array
  end
  if calculation[:add]
    add (numbers)
  else
    subtract(numbers)
  end
end