Consider a company that assembles computers. The probability of a faulty assembly of any computer is $p$. The company therefore subjects each computer to a testing process. This testing process gives the correct result for any computer with a probability of $q$. What is the probability of a computer being declared faulty?
answer = option A
in image below the ticks shows those branch where the result is declared as faulty.
so required probability = sum of those two branches $= pq + (1-p)(1-q)$