An exam has 5 problems, A, B, C, D and E. 52 students attend it. The result is shown as below:
The numbers of students who solve the problems correctly
Each student at least solves 1 problem correctly. 7 students solve 1 problem correctly, and 6 solve 5 problems correctly. The number of students who solve 2 problems correctly and those who solve 3 problems correctly is equal. How many students solve 4 problems correctly?