$(10\%)$ A computer whose processes have $4096$ pages in their address spaces keeps its page tables in memory. The overhead required for reading a word from the page table, is $500nsec.$ To reduce this overhead, the computer has a TLB$,$ which holds $128($page$,$ frame$)$ pairs and can do a lookup in $50$ nsec$.$ What hit rate is needed to reduce the mean overhead to $100nsec$ or less, assuming that the page table is consulted only if there is a miss?