roulette.rb

prng = Random.new
Trials = 37000
lost = 0
Trials.times do
  bad_results = 0
  250.times do
    result = prng.rand(1..37)
    if result <= 19
      bad_results += 1
    else
      bad_results = 0
    end
    if bad_results == 7
      lost += 1 
      break
    end
  end
end
puts "#{lost / Trials.to_f} probability of losing 7 in a row."

0.6873795620437956 probability of losing 7 in a row.