We know EMAT = H(t+m) + (1-H) (t+kn+m)
Here H = TLB hit ratio, t=TLB access time, n=Page table access time, m= Main memory access time, k = no. of levels of page tables.
Now simplifying the equation:-
EMAT = Ht + mH + t + m + n - Ht -mH - nH, Here k = 1,
After cancellation we get:-
= t + (1-H)n + m
But here we dont need to access the main memory, we only seek time required to read a word from the page table. So ignore m
So our formula boils down to t + (1-H)n
Putting the values 10ns + (1 - p) 50ns = 20ns . This gives 80% as the Hit ratio.
NOTE:- The questions on paging and TLB as done as:-
EMAT = Time taken to convert Virtual address to Physical Address(Address Translation) + Time to access the main memory
= t + (1-H) (k*n) + m + (PF*PFS)
where t=TLB access time, H=TLB hit ratio, k=no. of levels of paging, n = page table access time, m=main memory access time, PF= page fault rate, PFS= page fault service time.
We use the formula according to our requirement.
If the page fault is considered in the question , we use it otherwise we ignore it.