computer has a cache, main memory, and a disk. If a referenced word is in the cache, 20ns are required to access it. If it is in main memory but not in the cache (called cache miss), 100ns are needed to load it into the cache (this includes the time to originally check the cache), and then reference is started again. If the word is not in main memory, 10ms are required to fetch the word from disk, followed by 100ns to copy it to the cache, and then the reference is started again. The cache hit ratio is 0.90 and the main memory hit ratio is 0.75. What is the average time in ns required to access a referenced word on this system?