Acertain processor has access to three levels of memory. the access time of level 1 memory is 10 ns. the access time of level 2 memory is 50 ns. the access time of level 3 memory is 200 ns. assume the hit ratio of level 1 is 0.8. the hit ratio of level 2 is 0.95. if a word to be accessed is in level 1 memory, it can be accessed directly; if it is in level 2, then it is first transferred to level 1 memory and then accessed by the processer; if it is in level 3, it is first transferred from level 3 to level, then from level 2 to level 1, and then accessed by the processer. what is the average memory access time?