Probability of failing a test for A or B are $A=0.5$ and $B=0.2$.
Probability that at least one will fail is $0.5+0.2-(0.5)(0.2)=0.6$
Probability that both passing is $1-0.6=0.4$.
I thought I could get the probability of A and B passing by doing $1-A=0.5$ and $1-B=0.3$ and using the same equation on those numbers? Why isn't the probability of both passing $0.5+0.3-(0.5)(0.3)=0.65$? What's the problem in my logic of subtracting to get the probability of passing?