What is the value of (301+302+303+…+325) – (1+2+3+…+25)?

I don't think this would fall under permutation or combination. It's just an addition problem. Notice that you could write it as:

(300+1 + 300+2 + 300+3 + ... 300+25) - (1+2+3+...+25) or

(300+300+300+...+300) + (1+2+3+...+25) - (1+2+3+...+25)

See how we can now simplify it? We now just have 300 added together 25 times, or 300 x 25, since the sum of the numbers in the last two sets of parentheses are the same, and one is being subtracted from the other.

You could also write the problem this way, if it helps:

301-1 + 302-2 + 303-3 + ... + 325-25 = 300 x 25