A computer has a cache, main memory, and a disk used for virtual memory. If a referenced word is in the cache, 20 ns are required to access it. If it is in main memory but not in the cache, 60 ns are needed to load it into the cache (this includes the time to originally check the cache), then the reference is started again. If the word is not in main memory, 12 ms are required to fetch the word from disk, followed by 60 ns to copy it to the cache, and then the reference is started again. 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 word on this system? You don't need to compute the final numbers, just the formula is enough.

Please note that ns stands for nanosecond and ms for milliseconds and each millisecond = 1,000,000 nanosecond.

You can plug in your numbers in the formula, you don't need to simplify (no calculator needed)