At a shipping warehouse, a manager has purchased two warehouse automation robots, an autonomous mobile robot $($AMR$)$ and a pre$-$programmed aerial drone. The AMR can retrieve $21$ packages per hour, and its operating cost is $$3\mathrm{.}50$ per hour. The aerial drone can retrieve $28$ packages per hour, and its operating cost is $$5\mathrm{.}25$ per hour. For each hour of operation, the two robots require technicians to perform maintenance and calibration. The technicians usually spend $5$ minutes to perform maintenance on the AMR after each hour of operation, while the drone requires $12$ minutes of maintenance after each hour. The technicians also work on other warehouse machineries and can only spend $100$ minutes per day to maintain the two robots.