Write Through Method:
Write through is a storage method in which data is written into the cache and the corresponding main memory location at the same time. The cached data allows for fast retrieval on demand, while the same data in main memory ensures that nothing will get lost if a crash, power failure, or other system disruption occurs.
80%- requests for Read and 20%- requests for Write
So, according to definition we have to count (memory+cache) only for Read requests while for Write requests we just need to count (Main Memory)
0.80 [ (0.90) {hit ratio for cache} (10 ns) + (0.1) (100+10 ns) ] + 0.20(100 ns) = 36 ns
P.S.
Doubt-1: Why we taken Main Memory access time for Write Operaion instead of Taking 'Cache'
Doubt-2: What if here 80% Read Request not given?
Solutuion: (0.90) {hit ratio for cache} (10 ns) + (0.1) (100+10 ns) ???
Doubt-3: If they don't specify about method what we have to consider Strict Hierarchy or Simple one?
Doubt-4: What if here 'Write through' method was not used?