gistfile1.rb

puts "Type 'end' at any point to calculate the average."

marks = []
sum   = 0

while mark != "end"
  print "Enter Mark: "
  mark = gets.chomp
  break if mark == "end"

  begin
    mark = Float(mark)
    marks << mark
    sum = sum + mark
  rescue ArgumentError
    puts "Invalid mark #{mark}"
  end
end

average = sum / marks.size
puts "The Average grade of the class is #{average}"