I am stuck on this tutorial problem;
A number theory course is given in 3 different lecture theatres - ThA (9 seats per row), ThB (10 seats per row) and ThC (11 seats per row).
The students always fill the back rows first, so that the backmost rows are completely full, the frontmost rows are completely empty and at most 1 row is partially full.
The frontmost row has 5 students in ThA, 1 student in ThB and 10 students in ThC.
How many students are there? Assume that all students attended all lectures. You need to make a reasonable assumption about the size of the lecture theatres.