http://www.gooddinosaur.com/2009/11/male-female.html

family.rb

#!/usr/bin/env ruby
# usage: ./family.rb [trials]

# Family problem, 1000000 trials:
#
# 997496 girls, 1000000 boys
# 1.997496 children per family
# 0.997496 girls per boy
#
# Ran in 15.508096s

class Array
  def rand # A spoonful of sugar helps the medicine go down
    self[length*Kernel.rand]
  end
end

class Family
  def initialize(complete = true)
    @children = { :boy => 0, :girl => 0 }
    complete! if complete
  end
  
  def boys
    @children[:boy]
  end
  
  def girls
    @children[:girl]
  end
  
  def have_child!
    @children[[:boy, :girl].rand] += 1
  end
  
  def complete!
    have_child! until complete?
  end
  
  def complete?
    boys == 1
  end
end

iterations = if ARGV.length > 0
               ARGV[0].to_i
             else
               1000
             end

puts "Family problem, #{iterations} trials:"

started = Time.new

children = { :boys => 0, :girls => 0 }
iterations.times do
  f = Family.new
  children[:boys]  += f.boys
  children[:girls] += f.girls
end

family_size = (children[:boys] + children[:girls]).to_f / iterations
ratio = children[:girls].to_f / children[:boys]

elapsed = Time.new - started

puts
puts "#{children[:girls]} girls, #{children[:boys]} boys"
puts "#{family_size} children per family"
puts "#{ratio} girls per boy"
puts
puts "Ran in #{elapsed}s"