Given Hit Ratio for Reading = 60/100 = 3/5
∴ Miss Ratio m1 for Reading = 1-(3/5) = 2/5 = 0.4
Given access time of cache t1 = 30ns & main memory = 100ns
Average Memory Access Time for Reading = t1 + m1*t2 = 30 ns + (0.4 * 100 ns) = 70ns
Here, there nothing to do with 50% and access time for writing. Hence, all things are given in terms of reading only.