A computer whose processes have 4096 pages in their address spaces keeps its page tables in memory. The overhead for reading a word from a page table is 12 nsec. To reduce this overhead, the computer has a TLB, which holds 16 (virtual page, physical page frame) pairs, and can do a look up in 2 nsec. What hit rate is needed to reduce the mean overhead to 4 nsec? Make sure you show the equation that you solved (to support your answer).