Click the Apple logo top left of the screen and then click About This Mac.
1024
There are 1024 megabites in 1 gigabite. 20 gigabites has 20,480 megabites.
It depends on how many gigabites it has like 8 gigabites can hold about 15 movies.
None
5500
4500
4,000,000,000
A lot.
100
1.306728 gb
a half
Click on the apple logo at the top left of your screen. Click "About This Mac." Click "More Info..." After that an application should show up and click on "storage" then it should tell you how much you have left. Hope this answers your question.