CSS calc() allows you to mix and match units to get real-time calculations. It's useful when you need to size elements but you have some unknown number that you have to account for.
Hmm.. not sure what's up with the code tab, but here's a link:
Have a nice weekend
There are issues with the sound quality on your lessons. The volume goes up and down from lesson to lesson. I have my PC volume on max and this lesson (Create a fixed-fluid-fixed layout using CSS calc()) I can barely hear.