16.A fair die is tossed until a number grater than 4 apperars. The probability that an even number of tosses shall be required, is

17.The least number of times a fair coin must be tossed so that the probability of getting at least one head is at least 0.8 is

18.In an entrance test there are multiple choice questions. There are four possibnle answers to each question of which one is correct. The porbability that a student knows the answer to a question is 90 %. If he gets the correct answer to a question, then the probability that he was guessing is

19.A box contains 3 white and 2 red balls. If we draw one ball and without replacing the first ball, the probability of drawing red ball in the second draw is

20.A determinant is chosen at random from the set of all determinants of order 2 with elements 0 and 1 only. The probability that the determinant chosen is non-zero is