In a exam of 15 multiple choice questions, each student gets 3 points for each correct answer, 1 point is deducted for each incorrect answer, while he is not graded for each question that is not answered. If a student answers at least one question in the competition, the chances of getting a positive grade are:

72,41% or 83,25% or 86,74% or 91,75%

The probability is 1/3 for answering right, 1/3 for answering wrong and 1/3 for no answer