using_array.rb

array = []

loop do
  print "Enter Mark: "

  case mark = gets.chomp
  when Integer
    array << mark.to_f
  when "end"
    break
  else
    puts "Invalid mark #{mark}"
  end
end

sum = 0
array.each { |m| sum += m }
average = sum / array.size
puts "The Average grade of the class is #{average}"

using_counters.rb

sum = 0
num = 0

loop do
  print "Enter Mark: "

  case mark = gets.chomp
  when Integer
    sum += mark.to_f
    num += 1
  when "end"
    break
  else
    puts "Invalid mark #{mark}"
  end
end

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