Graph Algorithms · Challenge problem

Six tasks $p,q,r,s,t,u$ have prerequisites $p$ before $q$, $p$ before $r$, $q$ before $s$, $r$ before $s$, $r$ before $t$, $s$ before $u$ (and whatever these force transitively). For how many of the $\binom{6}{2}=15$ pairs of tasks is the relative order of the two tasks NOT forced – that is, either could legally be scheduled before the other?