Row Reduction & Rank · Hard problem

Four pipes carry flows $a,b,c,d$ (L/s, all required to be nonnegative) away from a pumping station, with $a+b=15$, $b+c=25$, and $c+d=45$. These three equations in four unknowns leave $d$ free. Among all choices of $d$ that keep every flow nonnegative, find the value of $b$ at the LARGEST feasible value of $d$.