Calculation Castle: Greco's Ghostly Challenge "Subtraction"

"The name's Greco. I'm sendin' you an invite to face the challenges that await in my castle! So, let's play a game!! I live at the very top of my castle! Do you got what it takes to reach me? It won't be easy. In order to get to me, you gotta pass through several rooms. In these rooms, you'll have to solve subtraction problems to pass. That's why my castle is called the Calculation Castle! Moreover, my castle is full of ghosts with many tricks standing in your way, so good luck and do your best! If you manage to beat my ghostly henchmen and get through the rooms, you'll receive lots of hidden rewards…but can you do it? I guess I'll wait and see! Heh heh heh…"