jmc

Sum the first two equations to find $c+o+n=t.$ Solve the third equation for $c$ to find $c=s-t,$ and substitute $s-t$ for $c$ in $c+o+n=t$ to find $o+n+s-t=t⟹o+n+s=2t.$ Substitute $12$ for $o+n+s$ to find $t=12/2=\boxed{6}.$