So with 24 student and 12 computers the order of which a student is placed at a computer doesn't matter. If a student has a computer to sit at they have a computer. So when we want to find the number of ways to do something when order doesn't not matter, we do a combination.
Which mean is expressed in your calculator as n C k = n!/ ( k! (n-k)! ) when n = 24 and k = 12.
So 24 C 12 = 24! / (12! (24-12)! )