Company A manufactures computer applications boards. They are concerned with the mean time before failures (MTBF), which they regularly measure. Denote the sample MTBF as ǔM and the true MTBF as μM. Determine the number of failures that must be measured before ǔM lies within 20 % of the true μM with a 90% probability. Assume the PDF is exponential, i. e fM(x) = (1 / µM) exp (– x / µM) u( x).