A computer has a cache, main memory, and a disk used for virtual memory. If a referenced

word is in the cacne, $2$U ns are required to access it$.$ IT It is in main memory but not in the cacne $60$ ns are needed to load it into the cache, and then the reference is started again. If the word is not in main memory, $12$ ns are required to fetch the word from the disk, followed by $60$ ns to

copv it to the cache, and then the reference is started aqain. The cache hit ratio is $0\mathrm{.}9$ and the

main memory hit ratio is $0\mathrm{.}6.$ What is the average time in nanoseconds required to access a

rererencea word on inis system $?$