here once we access memory then in these 200 ns(as given memory access time) the refreshing will be performed along with data access(read or write ) not both simultaneously.
in 1ms we are refreshing 64 times
in 200nsc we will refresh = 12.8 times
1 refresh operation takes 100ns
=> 12.8 refresh operation will take =12.8*100ns-(1)
among 200 ns that we are using for memory access above time is refreshment overhead
so % of memory access time used is= )(1)/200ns) *100=0.64