Fun, but relatively simple
SPOILER:
x = qty of soup
x/2 = qty of spaghetti
x/3 = qty of salad

x + x/2 + x/3 = 55
6x + 3x + 2x = 330
11x = 330
x = 30 → 30 bowls of soup, one per each camper.

The question states that only ONE switch controls the attic… So the warm lamp part doesn’t make sense, does it?

Here’s what I tought:
1) turn first switchon , if the light goes on, it’s the first one, else goto 2
2) turn second switch on, if the light goes on, it’s the second one, else goto 3
3) it’s the third one

So… where I am wrong? :)