A computer has a cache, main memory and disk (used for virtual memory). If a referenced word is in the cache, it requires 20 ns to access it. If it is in main memory, but not in the cache, then it requires 60 ns to load it into the cache, then pass on the word to the CPU. If the referenced word is not in the cache, or in main memory, but is on the disk, it requires 12 ms to fetch the work from the disk, then an additional 60 ns to copy it into the cache, then pass on the word to the CPU. The cache hit ratio is 0.9 and the main memory hit ratio is 0.6. a. What is the page fault ratio?