Question

In a multiple choice examination there are 20 question, each question has four alternative answer following it and a student must select one correct answer. Four marks are given for the correct answer and one mark is deducted for every wrong answer. A student must score at least 50% of maximum possible marks to pass the examination. Suppose a student has not study at all so he decide to answer the question on the random bases. What is the probability that he we pass the examination?