A disk drive manufacturer sells storage devices with capacities of one terabyte, 500 gigabytes, and 100 gigabytes with probabilities $0.5,0.3,$ and $0.2,$ respectively. The revenues associated with the sales in that year are estimated to be $\$ 50$ million, $\$ 25$ million, and $\$ 10$ million, respectively. Let $X$ denote the revenue of storage devices during that year. Determine the probability mass function of $X$.