CareerCup

a6 a5 a4 a3 a2 a1 a0
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--FA-- --FA--
(c1)(s1) (c2)(s2) %has the count of ones in respective input lines(3 each)

now add (a6 c1 s1) with (0 c2 s2) using HA and 2 FA to get the result

Use a0 a1 a2 and 1 FA to get c2 s2.
Use a5 a4 a3 and 1 FA to get c1 s1.
Which have the number of Ones in those six digits.
now add (a6 c1 s1) with (0 c2 s2) using HA and 2 FA to get the result

we hv to use half adders

15 half adders.
6 for a6 to a0 to get the LSB of result. adding all sums of half adders.
5 for carries from the previous 6 half adders to get the next result bit.
4 for carries from the previous 5 half adders to get the MSB of result

implement a 2 bit full synchronous adder; put A=B=0, Cin = Data(bit stream)
Cout S0 S1 will give desired result