Here memory hierarchy is followed , so for each main memory reference block need to be copied back to cache
So 1 word memory access time =120 ns
so 4 word memory access time=Tm=4*120=480 ns
Hc=cache hit ratio= 80%=0.8
(1-Hc)=memory hit ratio=cacge miss ratio=20%=0.2
Tc=20ns
Tm=480ns
Average Access time=Hc*TC + (1-Hc)*(Tm+Tc)=(0.8*20)+0.2*(480+20)=116ns