Case 1: The entry is in associative register it takes 200 ns to access the word memory. This happens 75% of time(w1). Let THIS 200 ns be c1.
Case 2: If the page is not in associative register --> 400 nanoseconds: 200 nanoseconds to access the page table and 200 nanoseconds to access the word in memory. This happens 25% of time(w2). Let this 400 ns be c2
Thus, effective access time is
= w1c1 + w2c2
= (0.75 × 200 ns) + (0.25 × 400 ns)
= 250 ns