Let TLB hit rate = x
So in case of TLB hit , only TLB will be accessed for address translation..In case of miss , assuming one level of paging , both TLB and page table will be accessed for address translation..
Hence effective address translation time = Time taken in hit + Time taken in case of miss
==> x(100) + (1-x)600 = 200
==> 600 - 500x = 200
==> x = 0.8
Hence required hit ratio of TLB = 0.8