20. (5) A computer has a cache, main memory, and a disk used for virtual memory. If a word is in the cache, 20 ns are required to access it. If it is in main memory, but not in cache, 60 ns are needed to load it into the cache. And then the reference is started again. If the word is not in main memory, 12 milliseconds (1 millisecond 108 ns) are required to fetch it from disk, followed by 60 ns to get it into the cache. If the cache hit ratio is 0.9, and the main memory hit ratio is 0.6, what is the average time in ns required to access a referenced w on this system? You must show how you arrived at your answer. If you just write down your final answer, you will not receive any credit. 21. (2) What is the function of a device driver