What does how much longer mean addition or subtraction

That would mean subtraction, as (example) 
Somebody has twenty inch paper. Another person has a ten inch paper. How much longer does that somebody's paper compare to another person's?
This means 20-10 = 10, so the answer to this is 10.

Hope this helps!

How much longer means subtraction, because how much longer is asking the length that one item is longer than the other, like a yardstick compared to a ruler, how much longer is the yardstick, something like that.

Hope this helps, and have a nice day!