We consider two queues with loss, one is the finite dam with Poisson arriva
ls and the other is the M/G/1 queue with impatient customers. We use the me
thod of Kolmogorov's backward differential equation and construct a type of
renewal equation to obtain the Laplace transform of busy(or wet) period in
both queues. As a consequence, we provide the explicit forms of expected b
usy periods.