Hi, In MS Excel there is a feature called Goal Seek. If you have a function and you need certain output from the function, you can ask goal seek to find the input for that function. The search for input value is done brute force style. I tried to imitate this feature in ruby when I wanted to calculate tax subsidy (tax subsidy is also taxed, tax calc utilizes tax brackets, non reversible, etc.) Here's my function: def subsidy_seek(net_salary, &net_if_subsidy) subsidy = 0 net_calc = net_if_subsidy.call(subsidy) increment = 65536 until net_salary - net_calc < 10 until net_if_subsidy.call(subsidy + increment) < net_salary increment /= 2 end subsidy += increment net_calc = net_if_subsidy.call(subsidy) end subsidy end It works, but it's a very specific function. Is there a ruby library for this kind of feature? Is there a mathematical term for goal seek? So if I want to learn more I can key in that term in google? Anybody know what kind of process goes behind goal seek? It seems to be able to handle most situations.