Weighted
Weighted helps you to pick items from a collection by it's weight/probability.
Inspired by Stackexchange solution and a (probably) nicer solution: Pickup
Installation
This was just a quick hack, too lazy to make a gem especially since Pickup is much more elegant.
Modify run.rb's call to Weighted.new and pass in a hash with items as the keys and occurances as the values.
Then just run run.rb:
ruby run.rb