carry look ahead generator has inputs P(i) =A(i) EXOR B(i) , Gi = A(i) AND B(i) , delay max(20,10) = 20
carry generation circuit C(i+1) = G(i) + C(i) P(i) , delay = 10 + 10 = 20
to generate sum S(i+1) = C(i) EXOR P(i) , delay= 20
Total delay to generate carry = 20+20 = 40 ns
Total delay to generate Sum = 20 +20 +20 = 60 ns