OK, I managed to collect all 6 parts. By the way - the last riddle is not logical in my opinion.
"The sum of two equal stones is 20, the difference between the largest and the smallest is 4." I think that correct answer is "10, 6, 10" (sum of two equal is 10+10=20, the difference between the largest and the smallest is 10-6=4) but program accepts "12, 8, 8" as a good answer.
There is also a bug in the arcade room. After
Attempt to open the lock I get a key. But later option Attempt to open the lock can be chosen again and this time I get a key too but game ends with game over.