A certain 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?

A desktop magnetic hard disk has an average seek time of 10ms and rotation speed of 7,200 rpm (Revolutions per Minute). An enterprise magnetic hard disk has an average seek time of 4ms and rotation speed of 15,000 rpm. How must faster is the enterprise hard disk in terms of the average access time?