TavgTotal = 0.7 * TavgR + 0.3 * TavgW
Now we have to calculate TavgR and TavgW
TavgR = 0.8 * Tc + 0.2 * Tm -----------------------( We are not doing 0.2*(Tc+Tm) becoz in look aside cache simultaneous cache and memory lookup is done. That means it will be max(Tc,Tm) in case of miss and min(Tc,Tm) in case of hit )
so TavgR = 0.8*20 + 0.2*100 = 36
TavgW = 0.7 * Tc + 0.3 * Tm ----------------------- ( Same reason here )
TavgW = 0.7*20 + 0.3*100 = 44
So, TavgTotal = 0.7*36 + 0.3*44 = 38.4