Information provided by the grader:

Homework 1:
 The only problem in which all students made a mistake is 1.5.16;
 here they all failed to prove why bs+r is in effect the remainder
 of the division.

Homework 6:
 You might want to tell the students that the computations in the first
 problem are very quick if done using the chinese remainder theorem.
 None of the students managed to do the last problem.