So we care about only the carry generated by FA because these act as inputs to the other FA.
So computing time for Carry C1,C2,C3,C4,C5,C6,C7.
we get 18+27+23+32+18+23+27 = 168 ns
So for last FA B2 we get compute sum also S8 = 23ns
seconds per addition = 168+23=191ns
additions per second = 1 / seconds per addition.
= 1 / 191ns = 5.23*10^6