A computer has a cache, main memory, and a disk used for virtual memory. If a referenced word is in the cache, 5 ns are required to access it. If it is in the main memory but not in the cache, 50 ns are needed to load it into the cache, and then the reference is started again. If the word is not in the main memory, 10 ms are required to fetch the word from the disk, followed by 50 ns to copy it to the cache, and then the reference is started again. The cache hit ratio is 0.8 and the main-memory hit ratio is 0.9. What is the average time required to access a referenced word on this system?