$(10\%)$ A compuler whose processes have $2048$ pages in their address spaces keeps its page tables in memory. The overhead required for reading a word from the page lable is $600nsec.$ To reduce this overhead, the computer has a TLB$,$ which holds $64($page$.$ frame$)$ pairs and can do a lookup in $100nsec.$ What hit rate is needed to reduce the mean overheald to $200$ nsec or less?