A computer technician notes that 40% of computers fail because of the hard drive, 20% fail due to the processor, 15% of problems are with the keyboard, and 5% of problems are due to something else. What is the mean of the number of computers he will work on before seeing his first computer that has failed because of the hard drive? Since there are four categories, the technician can expect 4 computers until one has failed due to the hard drive.
Since 40% is equivalent to Two-fifths, it is expected that Five-halves = 2.5 computers until one has failed due to the hard drive.
Since the highest percentage of failure is due to the hard drive, the technician can expect that the first computer will have failed due to the hard drive. So the expected value is 1.
Since it is more likely that a computer has failed due to some other issue, the technician can expect the first computer to fail due to some other issue, and the second to fail due to the hard drive. So the expected value is 2