Oh, is the part about the left scale weighing 120 lbs just another one of those useless pieces of information that isn't relevant to the puzzle?
Yeah haha. You could theoretically use the pounds to calculate the answer but it overcomplicates the puzzle.
In that case I'd consider rewording the rock clue as well, because that makes it sound like the "weight of the two scales" is incorporating that 120 lbs into the calculation.
