Very simple javascript calculator (using 3 variables)

We need to get a really simple web site based javascript application made that will take the numbers in 3 pulldowns users select and give the user a simple calculation back to them. We have some already finished coding we want to use and you'd just need to modify it to do the proper math sequence to give the right number. I don't think this project should take more than 3 hours at the very most and we're not looking to pay above $50 for the work.