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 [tex] \frac{1,890}{3,000} = \frac{63}{100} [/tex]
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 [tex] \frac{900}{3,000} + \frac{1,050}{3,000} = \frac{1,950}{3,000} = \frac{13}{20} [/tex]