JK Flip Flop , Counter Sequence?
The characteristic table for JK Flip flop is

J    K   Input(Qn)     output(Qn+1) 

0    0       Qn            Qn
0    1       X             0
1    0       X             1
1    1       Qn            !Qn


As for initial combination mentioned QA = 0 QB = 0 QC = 0

For JA =  QB = 0    KA = 1 =&gt; QAnext = 0
    JB = 'QA = 1    KB = 1 =&gt; QBnext = 1
    JC =  QB = 0    KC = 1 =&gt; QCnext = 0  


Hence we got 2. Now taking this combination of QA = 0 QB = 1 QC = 0; we can find the next output. In this way the process is repeated until an already produced output is obtained again so that a sequence is formed