Consider the following circuit of counter

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sir can you please explain the answer

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Characterstic Equation of JK flip-flop, $Q^+ = JQ'+K'Q$

$J_a=1,\;K_a=Q_b',\;Q_a^+=1.Q_a'+Q_bQ_a=Q_a'+Q_b$

$J_b=Q_c,\;K_b=1,\;Q_b^+=Q_c.Q_b'+0.Q_b=Q_c.Q_b'$

$J_c=1,\;K_c=Q_a',\;Q_c^+=1.Q_c'+Q_aQ_c=Q_c'+Q_a$

initial state is $S_4$ , means initially $Q_aQ_bQ_c$ is as 100

so sequential of states will be as 

Clock Pulse	$Q_aQ_bQ_c$
initially	$100$
CP$1$	$001$
CP$2$	$110$
CP$3$	$101$
CP$4$	$011$
CP$5$	$100$

So After $5$ clock pulses, we reach to initial states.

$1024$mod$5  = 4$ , it means after $1020$ clock pulses we reach to initial state and after $4$ more clock pulse it will be at $011$ means $S_3$

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Thank you sir