Part A:
The probability that a customer complained during the guaranteed period is given by the number of costumers that complained during the guarantee period devided by the total number of students.
Therefore, the probability that a customer complained during the guaranteed period is

Part B:
The probability that a customer complained about an electrical problem or a mechanical problem is given by the sum of the probability that the customer complained about an electrical problem or
the probability that the customer complained about a mechanical problem.
Therefore, the probability that a customer complained about an electrical problem or a mechanical problem is
