So When the i-th slack variable of the primal is not really zero, then the i-th variable of the twin is equivalent to zero. Also, If your j-th slack variable of the twin is just not zero, then the j-th variable in the primal is equal to zero. Listed here, https://emilianowpaul.dailyhitblog.com/33106396/find-someone-to-take-linear-programming-assignment-no-further-a-mystery