If P(A) = 0.72, what is P(A')?

A' is the complement of A; which means that if A is true, A' is false, and if A is false, A' is true.

A and A' cover all possibilities, with no overlap.

Thus P(A) + P(A') = 1.00.

So if P(A) = 0.72, P(A') = 1.00 - P(A) = 1.00 - 0.72 = 0.28.

thanks Geno, I haven't seen that notation.

This is the notation that I know as the complement of A

$$P(\bar A)$$