Graph Algorithms · Hard problem

A project has tasks $p,q,r,a,b,c$ with durations (days) and prerequisites $p \to q$ ($5$), $p \to r$ ($3$), $q \to a$ ($2$), $r \to a$ ($4$), $r \to b$ ($6$), $a \to c$ ($3$), $b \to c$ ($1$). Each number is how many days that task itself takes. By how many days does the whole project (ending at $c$) shorten if task $q \to a$'s portion is cut from $2$ days to $0$ days?