*16. The following problem arises in molecular biology. The surface of a bacterium is supposed to consist of several sites at which foreign molecules—some acceptable and some not—become attached. We consider a particular site and assume that molecules arrive at the site according to a Poisson process with parameter A. Among these molecules a proportion a is acceptable. Unacceptable molecules stay at the site for a length of time which is exponentially distributed with parameter ìé, whereas an acceptable molecule remains at the site for an exponential time with rate ì2. An arriving molecule will become attached only if the site is free of other molecules. What percentage of time is the site occupied with an acceptable

(unacceptable) molecule?