Calculation Castle: Greco's Ghostly Challenge "Subtraction" is a game that lets players enjoy studying while challenging their skills in math. It contains a collection of subtraction problems and math drills learned at the elementary level. There are up to 10 steps of learning, starting with elementary 1st grade level 1-digit math problems to developing skills such as borrowing numbers with 2-digit math problems and more. Advancing through the levels, players can naturally learn to solve 4-digit subtraction problems with ease. The game is composed of a series of stages and there are ghosts and locked doors within each stage. Greco and his ghosts will present the player with problems that they will need to solve in order.