given the main memory access time when the cache miss is =100ns
cache access time=10ns
and hit rate is =75%=0.75
there are 2 ways to find out memory access time
case1 : both cache and memory accessed simultaneously
then avg memory access time= (hit rate*cache access time)+(miss rate*main memory access time)
=(0.75*10)+((1-0.75)*100)=32.5ns
case 2: both cache and memory accessed serially
then avg memory access time=(hit rate*cache access time)+(miss rate*(cache access time+main memory access time)
=(0.75*10)+(0.25*(10+100))
=35ns