15 • □ = 15 14 + □ = 14 □ • 34 = 0 15 • □ = 0 18 - □ = 0 □ • 17 = 0

15 • 1 = 15 18 - 18 = 0 15 • 0 = 0 0 • 34 = 0 14 + □ = 14 0 • 17 = 0