Average Memory Access Time = 0.6 * Avg. time for read + 0.4 * Average Time for write
Main memory access time = 72 ns per word
Cache access time = Main memory access time/5 (Given in question) = 14.4 ns
Avg. time for read = 0.85 * 14.4 + 0.15(14.4+ * 72 * 4) (On a cache miss entire cache block is taken from main memory)
= 57.6 ns
(simultaneous memory organization is given in question for writing as all writes are going to memory irrespective of cache hit/miss. So, we can assume read is hierarchical access though it won't change the answer much)
Avg. time for write = 72 ns (No need to update the whole cache block- we are given main memory access time per word meaning a word can be directly accessed and there is no point updating the whole block in write through cache)
So, Average memory access time = 0.6 * 57.6 + 0.4 * 72 = 63.36 ns