Loved that game and every game in that series. Part of the fun was being dropped into this world with no clues and just having to feel your way around.
I admit that I surrendered after a while and got a walkthrough, which was followed by several hours of, “Oh for fucks sake I could have figured that out!”
Free clue: It’s not 15. That solution doesn’t work for the bottom numbers, and it’s very clearly stated that the 7 is not a typo.
Edit: What makes this puzzle so good is that it tempts you with an “obvious” solution that works with all but the last set, but there’s an equally obvious solution that does fit every set, if you can shake yourself free of the fist one.