Swedish Rounding module - Extends Numeric to allow you to do "swedish rounding" - see http://en.wikipedia.org/wiki/Swedish_rounding

extend_numeric_rounding.rb

# this module adds rounding to the nearest decimal number.
# by default it rounds to the nearest 10 pence
# but you can also round to the nearest 50 / 1000 or whatever..
#
module ExtendNumericRounding
 
  def roundup( nearest=10 )
    self % nearest == 0 ? self : self + nearest - ( self % nearest )
  end
 
  def rounddown( nearest=10 )
    self % nearest == 0 ? self : self - ( self % nearest )
  end
 
  def roundnearest( nearest=10 )
    up = roundup( nearest )
    down = rounddown( nearest )
    if ( up - self ) <= ( self - down )
      return up
    else
      return down
    end
  end
 
end
Numeric.send(:include, ExtendNumericRounding)

(Note default is to round to nearest 10, but you can specify to round to the nearest 100, 1000... etc. You can also use it to round to to any nearest number too..

Example using irb console:-

ree-1.8.7-2010.02 :001 > require "lib/extend_numeric_rounding"
=> true
ree-1.8.7-2010.02 :002 > 10.roundnearest
=> 10
ree-1.8.7-2010.02 :003 > 13.roundnearest
=> 10
ree-1.8.7-2010.02 :004 > 49.roundnearest
=> 50
ree-1.8.7-2010.02 :005 > 49.roundnearest(100)
=> 0
ree-1.8.7-2010.02 :006 > 50.roundnearest(100)
=> 100
ree-1.8.7-2010.02 :007 > 543.roundnearest(100)
=> 500
ree-1.8.7-2010.02 :013 > 543.roundnearest(5)
=> 545
ree-1.8.7-2010.02 :014 > 543.roundnearest(4)
=> 544
ree-1.8.7-2010.02 :015 > 543.roundnearest(3)
=> 543
ree-1.8.7-2010.02 :016 > 543.roundnearest(300)
=> 600
ree-1.8.7-2010.02 :017 > 543.roundnearest(320)
=> 640