The following table gives the type, cost per byte, and access time for the various types of memory available on a computer system:

Type of Memory	Cost	Access Time
SRAM	$25/MB	5ns
DRAM	$1/MB	60ns
HD	$10/GB	7ms

(a) For a memory system with 256KB of cache SRAM, 128KB of main memory DRAM, and 1GB of virtual memory (implemented as hard disk), what is the total cost of the memory system and the cost per byte?

(b) If the hit rate at each level in the memory hierarchy is 80%, what is the average memory access time?

(c) What is the average memory access time if the hit rate at each level except the last is 90%?

(d) How about if the hit rate is 99% at each level except the last?