Correct Answer - Option 4 : 28 ns
First a page is accesses in upper level memory, if it is present there then it is called as hit. But if page is not find in upper level memory, then it is called as miss, for this page is first copied from lower memory to upper level memory. During miss, it involves first access time for upper level memory then access time for lower level memory.
T1 is the memory access time for memory M1
T2 is the memory access time for memory M2.
H is the hit rate in M1.
So, 1- H is the hit rate in M2.
Average access time will be:
Hit × 10 + min × 100
= 0.8 × 10 + (1 - 0.8) × 100
= (8 + 20) nsec
= 28 nsec