In a fixed length reliability demonstration test carrying a 20% consumer's risk, the producer is being asked to demonstrate an MTBF of 2000 hours. The test is structured to "accept" the equipment if it has 4 failures (or less) in the test.
a) How long will the test take (maximum)?
b) What real MTBF does the equipment need for the producer to have an equal 20% risk (of not passing the test)?