Question 363334
The probability of giving a correct answer is 1/4, the probability for a wrong answer is 3/4.  If he has to give an answer, his expectation is {{{5*(1/4)+(-1)*(3/4) = 5/4 - 3/4 = 2/4 = 1/2}}}.  If he doesn't give an answer, his expectation is 0.  Therefore it is better to just guess the answer. =)