Create PHP calculator for website and teach me how to implement it on word press

I want to incorporate some calculators on my website. I want somebody to create simple php calculator and also show me step by step how to install and implement it on my wordpress website. I am using wordpress with shared linux hosting. I can find plenty of free calculator scripts online, but when i put them in my wp page (I have a php plugin installed: Exec-PHP) the calculator does not execute properly. (either nothing happens or the page refreshes back to the homepage). I want somebody to write a basic script, or find one online; then show me exactly step by step how you put it into wordpress pages so that I can make additional different calculators myself. Once you have given me working calculator script and a plan of how to properly make it work on wordpress, i will edit the script myself to make different calculators that I will put on my site later.