Here the overhead is reading a word from the page table and we need to reduce the overhead.
So , when access is sequential memory access then
(new mean ovehead time) = TLB hit rate (TLB access time) + TLB miss rate (TLB access time +Page table access time )
200 = h(100)+(1-h)(100+500)
h=0.8
when access is parallel memory access then
(new mean ovehead time) = TLB hit rate (TLB access time) + TLB miss rate (Page table access time )
200 = h(100)+(1-h)(500)
h=0.75