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3 Answers

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Total delay will be due to 3 levels as above

  • P&G generator
  • Carry generator
  • Sum Generator

P&G generator has a single level of EX-OR & AND gates they will work in parallel 

delay1 =max(20,10) =20

Carry generator has a two levels  AND + OR  gates they will work one after another 

delay2 =10+10 =20

Sum generator has a single level of EX-OR gates they will work all  in parallel 

delay3 =20

Total delay  =delay1+delay2+delay3

                     20 + 20 +20 = 60ns

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If you Know the Structure of Carry LookaHead Genrator Then it Is Easy

Propagation delay = Max( 20,10) + 10 +10 + 20

                                = 20+10 +10+20

                                 = 40

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