The other one is also a girl.

gistfile1.rb

# Ruby
# Suppose Mr. and Mrs. Smith tell you they have two children, one of 
# whom is a girl. What is the probability that the other is a girl?
# http://www.whydomath.org/Reading_Room_Material/ian_stewart/9609.html
1.upto(10) do |n|
  siblings = []
  # Create a bunch of randomized pairs of children
  1.upto(10000) do 
    child1 = rand(2) == 0 ? :boy : :girl 
    child2 = rand(2) == 0 ? :boy : :girl 
    siblings << { :first_born => child1, :second_born => child2 }
  end 

  # Remove all [:boy, :boy] because we know one of them is a girl
  siblings = siblings.select {|ch| !(ch[:first_born] == :boy && ch[:second_born] == :boy) } 

  # Count the number of [:girl, :girl] because that's the combination
  # that is the answer to the question.
  girl_girl_count = siblings.select{|ch| ch[:first_born] == :girl && ch[:second_born] == :girl}.length

  puts "Study #{n}"
  puts "The other is a girl in #{girl_girl_count} cases."
  puts "Total number of cases #{siblings.length}." 
  puts "Chance #{girl_girl_count.to_f / siblings.length}"
  puts
end