create or replace function percent_passed_works(tutor number)
return number
is all_works number;
   passed_works number;
begin
    select count(grade)
    into all_works
    from grades
    where tutor_id = tutor;

    select count(grade)
    into passed_works
    from grades
    where grade > 60;

    return (passed_works / all_works ) * 100;
end;