Question 3: First of all, A and B are mutually exclusive events and can't happen at the same time, so the probability of A and B is P(A)+P(B)=0.2+O.6=0.8.

Question 4: Look at the denominator first. The part in the radical sign should be greater than or equal to 0, so 2-x >; =0 and since the denominator cannot be equal to 0, 2-x >; 0, so I want to x; 0, x< 1. In summary, the value range of x is (1, 2), and B is selected.