The marks of 50 students are given as a 'more than' table. More than 0: 50, more than 10: 46, more than 20: 40, more than 30: 20, more than 40: 8, more than 50: 0. How many students scored marks lying in the class 30–40?

In a 'more than' table the frequency of a class is the difference of the entries at its two end points: (more than 30) − (more than 40) = 20 − 8 = 12 students. The entry 20 on its own counts everyone above 30, including the 8 students who scored above 40.