attr_accessor

class Thing
  attr_accessor :quantity
  def initialize
    @quantity = 100
  end

  def decrement(qty)
    difference = quantity - qty
    quantity = difference
  end
end

irb(main):012:0> thing = Thing.new
=> #<Thing:0x007fc84b2ed928 @quantity=100>
irb(main):013:0> thing.quantity
=> 100
irb(main):014:0> thing.decrement(10)
=> 90
irb(main):015:0> thing.quantity
=> 100