Correct Answer - Option 3 : 60 ns
Concept:
TM = h × TC + (1 – h) × TP
TM is the average memory access time
TC is the cache access time
TP is the access time for physical memory
h is the hit ratio
Calculation:
Given that TC = 10 ns, TM = 20 ns, h = 80% = 0.8
TM = h × TC + (1 – h) × TP
⇒ 20 = 0.8 × 10 + (1 – 0.8) × TP
⇒ 20 = 8 + 0.2 TP
⇒ TP = 60 ns