Consider a process executing on an operating system that uses demand paging

Q. Consider a process executing on an operating system that uses demand paging. The average time for a memory access in the system is M units if the corresponding memory page is available in memory, and D units if the memory access causes a page fault. It has been experimentally measured that the average time taken for a memory access in the process is X units.

Which one of the following is the correct expression for the page fault rate experienced by the process?

(A) (D – M) / (X – M)                                         (B) (X – M) / (D – M)

(C) (D – X) / (D – M)                                         (D) (X – M) / (D – X)

Ans:  (X – M) / (D – M)