Use the two$-$level memory equation in Appendix $1$A to answer this question: A computer has a cache and main 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$),$ and then the reference is started again. The cache hit ratio is $0\mathrm{.}9.$ What is the average time in ns required to access a referenced word on this system?