Use method_missing hook to define custom methods on Numeric => ex: 3.rupees.in(:euros)

converter.rb

class Numeric
  
  @@currencies = {'yen' => 0.013, 'euro' => 1.292, 'rupee' => 0.019, 'dollar' => 1.0}
  
  def method_missing( method_id, *args, &block )
  
    singular_ccy = method_id.to_s.gsub( /s$/, '')
    
    if @@currencies.has_key?( singular_ccy )
      self * @@currencies[singular_ccy] # Convert into dollars   
    else
      super
    end
       
  end
  
  def in( currency )
    singular_ccy = currency.to_s.gsub( /s$/, '')
    self / @@currencies[singular_ccy]  if @@currencies.has_key?( singular_ccy )
  end
  
end