what will be the average instruction time?

Question

If a page fault occurs on an average every 1000 instructions and average instruction time is 100ns without page fault and 100ms with page fault. what is average instruction time?

Answer 1

Page fault rate = 1/1000 
T =Page fault rate * Page Fault Service Time + (1-Page Fault rate)*Memory access time
  = $\frac{1}{1000} * 100 * 10 ^6$+  $(1- \frac{1}{1000}) * 100$  
  = 100099.9 ns