In a certain grade, there was a test of three problems A, B and C.
The percentage of the students who gave the right answer of A was 60%, who gave the right answer of B was 70% and who gave the right answer of C only was 15%.
Moreover, those who gave the right answer of A and did not give right answer of B were 18 persons.
Those who did not give right answers of all three problems were 5%.
Find the number of students of this grade.