Question

the probability of guessing the correct answer to a certain question is x/2. If the probabilityof not guessing the correct answer to the question is 2/3, then find x

Answer 1

The sum of all the probabilities is equal to 1 
So,  x/2 = 1 - 2/3
 x/2 = 1/3
x = 2/3 

Answer 2

We have p(A)+p(A`)=1
given p(A)=x/2
and p(A`)=2/3
x/2+2/3=1
x=2/3