Show me the steps to solve A computer has registers, cache, main memory and a disk used for virtual memory. If a word to be accessed is found in the registers, $20$ microseconds are required to access it by the Arithmetic Logic Unit. If it is in cache but not in the registers, $40$ microseconds are required to bring it to the registers, then the reference is started again. If the word is not in cache, $30$ milliseconds are required to fetch the word from main memory, followed by $40$ microseconds to copy it to the registers, then the reference is started again. If the word is not in main memory, $720$ milliseconds are required to fetch the word from disk, followed by $30$ milliseconds to get the word to cache, followed by $40$ microseconds to copy it to the registers, then the reference is started again. Kindly ignore the time required for the processor to determine whether the word is in registers, cache, main memory or disk. Also, if the word is not found in the registers, cache or main memory then it has to be compulsorily present in the external disk. If the registers, cache and main memory have a hit ratio of $0\mathrm{.}8,0\mathrm{.}5$ and $0\mathrm{.}2$ respectively, what is the average time in microseconds required to access a referenced word on this entire system?