Dats Mathematics

prof : how much 7 x 24 = ?
student : it's 168.
prof : prove it.
student: 16 + 8 = 24
prof : and 7 x 27 + ?
student : 189 prove 18+9 = 27
prof : and 21 x 7 = ?
student : 147 prove 14 + 7 = 21
prof: and 18 x 7 = ?
student : 126 prove 12 + 6 = 18
etc..