def get_grade(array)
  sum = 0
    array.each do |x|
    sum += x
  end
  average = sum / array.length
  case average
    when 90...100
      grade = 'A'
    when 80...90
      grade = 'B'
    when 70...80
      grade = 'C'
    when 60...70
      grade = 'D'
    when 50...60
      grade = 'E'
    when 0...50
      grade = 'F'
    else 
      "Error"
  end
end