For maximum efficiency hit ratio should be 1.
when H=1, then we need 3 memory access, two m/m access to access the two-level page tables, and one more m/m access to access the physical memory.
So, maximum efficiency when H=1, E.M.A.T = $3*10^-8$ or 30ns
Efficiency should be 80% from 100%.
When efficiency is 100% then E.M.A.T is 30ns
When efficiency is 80% then E.M.A.T will be 30ns/.80 = 37.5 ns (80% of 37.5ns is 30ns)
Now calculate the value of H:
37.5ns = H(30ns) + (1-H)($10^{-3}$)
(or)
37.5ns = H(30ns) + (1-H)($10^6$ns)
now all time units are same.
37.5 = H(30) + (1-H)($10^6$)
37.5 = 30H + $10^6$ -H$10^6$
H = $\frac{999962.5}{9999970}$ = 99.99%