Hierarchical access to main memory..
Tc : Cache Access Time, Tm : Memory Access time, Tm = 20Tc, Hit Ratio = 0.80
Effective access Time = Tc + 0.20 *Tm = Tc + 0.20*20Tc = 5*Tc
Speed Up = Memory Access Time Without Cache / Memory Access Time With Cache
= Tm/Effective Access time = 20*Tc/5*Tc
Speed up = 4