# first, in a lib file I wrote a quick function for gsubbing multiple
# items in a hash
class Object::String
  def gsub_from_hash(somehash = {})
    temp = self
    for part in somehash
      unless part[0].nil? or part[1].nil?
        temp = temp.gsub(part[0], part[1])
      end
    end
    temp
  end
end



# say I have a field called some_expression,
# and after submitting a form, we have:
# my_object.some_expression = "($A + $B) * $C"


#Then you can gsub the values for your variables:
real_formula = my_object.some_expression.gsub_from_hash({'$A' => '6', '$B' => '9', '$C' => '4' })
# note that I had to pass the numbers as String objects -
# the gsub_with_hash function would need further tweaking to get it
# to accept both strings and numbers

# now real_formula looks like "(6 + 9) * 4"
# since this is a valid mathmatical operation, you can use eval

answer = eval real_forumla

# answer => 60