Hello! I am so confused with these questions. Could you please help me? Thank you!

20. (8pts) Consider a random variable X and its (partially known) probability mass function ?, if x = 0 0.5, if x = 1 f(x) = 0.1, if x = 3 ?, if x = 4 0, otherwise The mean of the above distribution is known to be 0.8 (i.e., E(X) = 0.8). Then f(0) = and f(4) = Save the final result up to 2 decimal points.23. (4pts) How many 3-digit numbers can be formed from the digits 2, 3, 5, 6, 8 and 9, which are divisible by 5 (so the last digit must be 5) and none of the digits is repeated