Average instruction execution time = CPU time + Memory access time

Memory access time = No. of memory accesses by the instruction * average memory access time

Average memory access time =
 [probability of a TLB hit * regular memory access time]
 + [probability of a TLB miss * (Time for accessing the 1st level of the paging tables
     + Time for accessing the 2nd level of the paging tables
     + Time for accessing memory which may not be paged in with the translated address)]

Time for accessing the 1st level of the paging tables
= Time for accessing the 2nd level of the paging tables
= Time for accessing memory which may not be paged in with the translated address
    = [probability of a page fault * (page fault service time + regular memory access time) ]
      + [probability of no page fault * regular memory access time]
    = [0.0001 * (8 ms + 150ns)]  + [0.9999 * 150 ns]
    = 950 ns

Thus, average time per memory access = [0.9 * 150ns] + [0.1 * 3 * 950ns] = 420ns

Average instruction execution time = 100ns + (2 * 420ns) = 940ns