cache access time (tc) = 10ns
MM access time (tm) = 100ns
Block size = 2W
Accessing 2 Words from MM = 2*100 = 200ns
Tavg read= 0.8*10 + 0.2* (200+10) =50ns
{simultaneous writing time Max(Tc,Tm) = 100ns}
Tavg write = 0.9 * 100 + 0.1( 200 +100) = 120ns {write allocate policy}
as given,
20% references are for write operations, means 80% are read
Tavg=0.8 * 50ns + 0.2* 120ns
=64ns