ignoring search time with in the cache ==> this statement tells us to use parallel access.
$T_{avg\;access} = H_1.T_1 + \color{red}{(}1-H_1 \color{red}{)} \color{red}{(} \color{green}{H_2 . T_2 + (1-H_2).T_M} \color{red}{)}$
$T_{avg\;access} = (0.7*0.5) + \color{red}{(}0.3\color{red}{)} \color{red}{(} \color{green}{ (0.8*5) + (0.2).100} \color{red}{)}$
$T_{avg\;access} = (0.35) + \color{red}{(}0.3\color{red}{)} \color{red}{(} \color{green}{ 4 + 20} \color{red}{)}$
$T_{avg\;access} = 0.35 + 7.2 = 7.55\;ns$