**If nothing is mentioned take fan-in for a logic gate = 2 inputs and they will say in the question whether to take half adder or full adder for the 1st stage of LSB addition(ripple carry adder).**

Propagation delay In carry look-ahead adder(n bit adder):-

Carry look-ahead adder works in three phases:-

**In ripple carry adder all Pi, Gi terms are computed simultaneously after that all carry terms are computed simultaneously and after that all sum terms are computed simultaneously.**

The first level will take 2 time unit to generate all Pi, Gi terms using AND-OR gate in 2 levels only if variables are given in both complemented and uncomplemented form.

similarly 2nd level will also take further 2 time unit for generating the all carry terms.

and after that to produce the sum terms it will take further 2 time unit. so total propagation delay is 6 time unit.

**Note:- Any simple or complex logical function can be written in SUM OF PRODUCT form and can be realized using 2-levels of AND, OR gate only if both complemented and uncomplemented forms are given.**

**Now lets extend this question further for Ripple carry adder:-**

In ripple carry adder(n bit adder):- Assume Full adder is used for LSB computation, not half adder

Tpropagation =(n-1)* T.carry generation time by one full adder+ max(T.carry, T.sum)

= 3 * 2 (2 because C=AB+BC+AC can be implemented in 2 levels of AND, OR gate) + max(2, 4)

T.sum=4, because A⊕B can be implemented using 2 levels of AND, OR gate only if variables are given in both complemented and uncomplemented form as mentioned above. then further one more XOR with C. so total 4 level of AND, OR gate.

Tpropagation for ripple carry adder = 10

for more-https://www.gatevidyalay.com/delay-in-ripple-carry-adder/