For the product testing example in Problem 1, what is the mean time between failures (MTBF) for the products?

Data From Problem 1

‘We have a test bank where we test batches of 100 of our products continuously for seven days and nights. This week only three failed, the first after 10 hours, the second after 72 hours and the third after 1020 hours.’ What is the failure rate in percentage terms and in time terms for this product?