Diagnostic Test In an angiogram test, the doctor injects a dye into your artery and then performs an $X$ -ray to view blockages. Assuming that the dye is excreted after a time $T$ (in hours) which is
a random variable that is exponentially distributed with $a=1 / 3$
(a) Find the mean time for the dye to be excreted.
(b) Find the probability that the time for the dye to be excreted
is less than 90 minutes.