Suppose that a large file is being accessed by a computer memory system comprising of a cache and a main memory. The cache access time is 60 ns. Time to access main memory (including cache access) is 300 ns. The file can be opened either in read or in write mode. A write operation involves accessing both main memory and the cache (write-through cache). A read operation accesses either only the cache or both the cache and main memory depending upon whether the access word is found in the cache or not. It is estimated that read operations comprise of 80% of all operations. If the cache hit ratio for read operations is 0.9, what is the average access time of this system?