Question:
The probability of guessing the correct answer to a certain test questions is $\frac{x}{12}$. If the probability of not guessing the correct answer to this question is $\frac{2}{3}$, then $x=$
(a) 2
(b) 3
(c) 4
(d) 6
Solution:
GIVEN: Probability of guessing a correct answer to a certain question is 
Probability of not guessing a correct answer to a same question 
TO FIND: The value of x
CALCULATION: We know that sum of probability of occurrence of an event and probability of non occurrence of an event is 1.
If E is an event of occurrence and 
is its complementary then
$P(E)+P(\bar{E})=1$
According to the question we have
$\frac{x}{12}+\frac{2}{3}=1$
$\frac{x+8}{12}=1$
$x+8=12$
$x=4$
Hence the correct option is $(c)$