Let 'a' and 'b' the only outcomes of an event.
The probability of occurring outcomes 'a' is written as p(a) and
The probability of occurring outcomes 'b' is written as p(b)
The sum of probability of these event can be written as, p(a) +p(b)= 1
Here p(a)=0.73 and we need to find p(b)
We know that p(a) +p(b)= 1
0.73 + p(b)= 1
p(b) = 1 - 0.73 = 0.27
therefore the probability of b, p(b)=0.27