Malik, S. ., and Upma. “COST-BENEFIT ANALYSIS OF A SYSTEM OF NON- IDENTICAL UNITS UNDER PREVENTIVE MAINTENANCE AND REPLACEMENT”. Journal of Reliability and Statistical Studies, vol. 9, no. 02, Aug. 2020, pp. 17-27, https://journals.riverpublishers.com/index.php/JRSS/article/view/20991.