In a multiple choice test consisting of 90 questions. Each question has four choices out of which 1 is correct. +4 for each correct and -1 for each incorrect answer. Should the optimal strategy be to guess all the answers one doesn't know as guessing more answer would get us closer to the expected value or should one play safe and not guess at all or guessing a certain number of questions would be optimal?
My attempt: By expected value, for each question: 4*1/4 - 1*3/4 = +1/4. Therefore for all 90 questions expected score would be +22.5 points. Best strategy should be to guess all. I don't know. Help.