“…A number of very interesting problems arising in the theories of inventory, stock control, queuing theory, reliability, mathematical insurance, stochastic finance etc., can be expressed by means of renewal processes, renewal-reward processes, random walk processes and their modifications. There are many important theoretical results about these subjects in literature (Borovkov (1984), Brown and Solomon (1975), Feller (1971), Gihman and Skorohod (1975), Janssen and Leuwaarden (2007), Khaniyev et al (2008Khaniyev et al ( , 2013, Lotov (1996), Rogozin (1964), etc.). The results of these studies have complex mathematical structures and they are not useful for applied problems.…”