Help writing a simple grade calculator

I hope I'm posting this to the right place.

I'm trying to write a simple grade calculator for my students to post on my webpage...

Basically it should look like this:

What is your goal mark (%)? ________

What is your final term mark (%)? ________

_______% is what you need to score on the final exam to reach your goal mark.

Students should be able to fill in the first two blanks and click a calculate button so the script can fill in the third blank.

The script needs to calculate the mark this way:

((goal mark - term mark)*3)+term mark)=exam mark

Thanks for your help!