Redundancy allocation problem (RAP) is one of the most important issues in reliability engineering to increase the reliability of a system. The design of the redundant systems involves determining the redundancy strategies, redundancy levels, the type of components, and system configuration. The redundancy strategy can be active, standby, or active/standby. This study for the first time provides an approach to determine the optimal configuration in k-out-of-n repairable systems with mixed active/cold standby redundancy strategy to simultaneously maximize system availability and minimize system cost. To deal with this combinatorial problem, a Non-Dominated Sorting Genetic Algorithm II (NSGA-II) is employed. To find the most suitable solutions amongst Pareto solutions, a multi-criteria decision making method, TOPSIS, is employed. To validate the performance of the proposed method for systems with mixed active/standby redundancy strategies, the results are compared with the results of applying the proposed method to systems with active strategy and also systems with cold standby strategy. The results show that both cold standby and mixed redundancy strategies provide high levels of availability. Using mixed redundancy strategy is preferred over the active and cold standby counterparts, because it delivers design flexibility and provides maximum availability. Interestingly, active components used in the proposed mixed strategy do not impose extra costs.