AQA D2 2016 June — Question 6 14 marks

Exam BoardAQA
ModuleD2 (Decision Mathematics 2)
Year2016
SessionJune
Marks14
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicNetwork Flows
TypeLower and upper capacity networks
DifficultyStandard +0.3 This is a standard Decision Mathematics 2 network flows question covering routine algorithms (cut calculation, feasible flows, flow augmentation). While multi-part with several steps, it follows textbook procedures without requiring novel insight. The lower/upper capacity constraint adds minor complexity but remains algorithmic. Slightly easier than average A-level due to its procedural nature.
Spec7.02r Graph modelling: model and solve problems using graphs7.04f Network problems: choosing appropriate algorithm

6 The network shows a system of pipes with lower and upper capacities for each pipe in litres per second. \includegraphics[max width=\textwidth, alt={}, center]{34de3f03-a275-44fb-88b2-b88038bcec97-22_817_744_397_648}
    1. Find the value of the cut \(X\).
    2. Hence state what can be deduced about the maximum flow from \(A\) to \(H\).
  1. Figure 3 shows a partially completed diagram for a feasible flow of 28 litres per second from \(A\) to \(H\). Indicate, on Figure 3, the flows along the edges \(B D , B E\) and \(C D\).
    1. Using your feasible flow from part (b) as an initial flow, indicate potential increases and decreases of the flow along each edge on Figure 4.
    2. Use flow augmentation on Figure 4 to find the maximum flow from \(A\) to \(H\). You should indicate any flow augmenting paths in the table and modify the potential increases and decreases of the flow on the network.
    3. State the maximum flow and indicate a maximum flow on Figure 5. \section*{Answer space for question 6} \begin{figure}[h]
      \captionsetup{labelformat=empty} \caption{Figure 3} \includegraphics[alt={},max width=\textwidth]{34de3f03-a275-44fb-88b2-b88038bcec97-23_682_689_312_397}
      \end{figure} \begin{figure}[h]
      \captionsetup{labelformat=empty} \caption{Figure 4} \includegraphics[alt={},max width=\textwidth]{34de3f03-a275-44fb-88b2-b88038bcec97-23_935_1477_1037_365}
      \end{figure} Figure 5
      \includegraphics[max width=\textwidth, alt={}]{34de3f03-a275-44fb-88b2-b88038bcec97-24_2032_1707_219_153}

AnswerMarks Guidance
AnswerMarks Guidance
45B1
\(\le 45\)B1F oe in words
\(BD = 4\), \(BE = 4\), \(CD = 6\)B1, B1, B1
Correct at least one of \(AB, AC, AD, DH\) including directions, shown on diagramM1
All correct at \(AB, AC, AD, DH\) including directions, shown on diagramA1
All correctA1
Modifying one feasible flow correctly on diagram, must have scored M1 in part (i)B1 Augmenting both increases and decreases on one flow
eg: One correct flow in table; Second flow correct in tableM1, A1
All correctA1
[Max flow =] 32. Diagram must have \(AB = 12, AC = 11, AD = 9, GH + EH + BH + DH + FH = 32\). Different possibilities for other edgesB1
All correctB1
Question 2 (Alternative Answers)
Additional acceptable row reduction forms:
\[\begin{array}{ccccc}
x & 1 & 2 & 2 & 0 \\
1 & 0 & 1 & 0 & 0 \\
3 & x & 2 & 1 & 0 \\
0 & -1 & 0 & 0 & 0 \\
0 & -x & 0 & 0 & -1
\end{array} \text{ or } \begin{array}{ccccc}
x & 0 & 1 & 2 & 1 \\
1 & 0 & 1 & 1 & 2 \\
1 & x & 0 & 0 & 0 \\
0 & 1 & 0 & 1 & 2 \\
0 & x & 0 & 1 & 3
\end{array} \text{ or } \begin{array}{ccccc}
x & 0 & 1 & 2 & 1 \\
1 & 0 & 1 & 1 & 2 \\
1 & x & 0 & 0 & 0 \\
0 & -1 & 0 & 1 & 2 \\
0 & x & 0 & 1 & 3
\end{array}\]
Further intermediate forms shown as:
\[\begin{array}{ccccc}
-x & 0 & 1 & 1 & 0 \\
-1 & 0 & 1 & 0 & 1 \\
2 & x & 1 & 0 & 0 \\
0 & -1 & 0 & 0 & 1 \\
0 & -x & 0 & 0 & 2
\end{array} \text{ or } \begin{array}{ccccc}
-x & 0 & 1 & 1 & 0 \\
-1 & 0 & 1 & 0 & 1 \\
2 & x & 1 & 0 & 0 \\
0 & -1 & 0 & 0 & 1 \\
0 & -x & 0 & 0 & 2
\end{array} \text{ or } \begin{array}{ccccc}
-x & 0 & 0 & 1 & 0 \\
-0 & 0 & 0 & 0 & 1 \\
1 & x & 0 & 0 & 0 \\
0 & 2 & 0 & 1 & 2 \\
-0 & x & 0 & 1 & 3
\end{array}\]
| Answer | Marks | Guidance |
|--------|-------|----------|
| 45 | B1 | |
| $\le 45$ | B1F | oe in words |
| $BD = 4$, $BE = 4$, $CD = 6$ | B1, B1, B1 | |
| Correct at least one of $AB, AC, AD, DH$ including directions, shown on diagram | M1 | |
| All correct at $AB, AC, AD, DH$ including directions, shown on diagram | A1 | |
| All correct | A1 | |
| Modifying one feasible flow correctly on diagram, must have scored M1 in part (i) | B1 | Augmenting both increases and decreases on one flow |
| eg: One correct flow in table; Second flow correct in table | M1, A1 | |
| All correct | A1 | |
| [Max flow =] 32. Diagram must have $AB = 12, AC = 11, AD = 9, GH + EH + BH + DH + FH = 32$. Different possibilities for other edges | B1 | |
| All correct | B1 | |

# Question 2 (Alternative Answers)

Additional acceptable row reduction forms:

$$\begin{array}{ccccc}
x & 1 & 2 & 2 & 0 \\
1 & 0 & 1 & 0 & 0 \\
3 & x & 2 & 1 & 0 \\
0 & -1 & 0 & 0 & 0 \\
0 & -x & 0 & 0 & -1
\end{array} \text{ or } \begin{array}{ccccc}
x & 0 & 1 & 2 & 1 \\
1 & 0 & 1 & 1 & 2 \\
1 & x & 0 & 0 & 0 \\
0 & 1 & 0 & 1 & 2 \\
0 & x & 0 & 1 & 3
\end{array} \text{ or } \begin{array}{ccccc}
x & 0 & 1 & 2 & 1 \\
1 & 0 & 1 & 1 & 2 \\
1 & x & 0 & 0 & 0 \\
0 & -1 & 0 & 1 & 2 \\
0 & x & 0 & 1 & 3
\end{array}$$

Further intermediate forms shown as:

$$\begin{array}{ccccc}
-x & 0 & 1 & 1 & 0 \\
-1 & 0 & 1 & 0 & 1 \\
2 & x & 1 & 0 & 0 \\
0 & -1 & 0 & 0 & 1 \\
0 & -x & 0 & 0 & 2
\end{array} \text{ or } \begin{array}{ccccc}
-x & 0 & 1 & 1 & 0 \\
-1 & 0 & 1 & 0 & 1 \\
2 & x & 1 & 0 & 0 \\
0 & -1 & 0 & 0 & 1 \\
0 & -x & 0 & 0 & 2
\end{array} \text{ or } \begin{array}{ccccc}
-x & 0 & 0 & 1 & 0 \\
-0 & 0 & 0 & 0 & 1 \\
1 & x & 0 & 0 & 0 \\
0 & 2 & 0 & 1 & 2 \\
-0 & x & 0 & 1 & 3
\end{array}$$
6 The network shows a system of pipes with lower and upper capacities for each pipe in litres per second.\\
\includegraphics[max width=\textwidth, alt={}, center]{34de3f03-a275-44fb-88b2-b88038bcec97-22_817_744_397_648}
\begin{enumerate}[label=(\alph*)]
\item \begin{enumerate}[label=(\roman*)]
\item Find the value of the cut $X$.
\item Hence state what can be deduced about the maximum flow from $A$ to $H$.
\end{enumerate}\item Figure 3 shows a partially completed diagram for a feasible flow of 28 litres per second from $A$ to $H$.

Indicate, on Figure 3, the flows along the edges $B D , B E$ and $C D$.
\item \begin{enumerate}[label=(\roman*)]
\item Using your feasible flow from part (b) as an initial flow, indicate potential increases and decreases of the flow along each edge on Figure 4.
\item Use flow augmentation on Figure 4 to find the maximum flow from $A$ to $H$. You should indicate any flow augmenting paths in the table and modify the potential increases and decreases of the flow on the network.
\item State the maximum flow and indicate a maximum flow on Figure 5.

\section*{Answer space for question 6}
\begin{figure}[h]
\begin{center}
\captionsetup{labelformat=empty}
\caption{Figure 3}
  \includegraphics[alt={},max width=\textwidth]{34de3f03-a275-44fb-88b2-b88038bcec97-23_682_689_312_397}
\end{center}
\end{figure}

\begin{figure}[h]
\begin{center}
\captionsetup{labelformat=empty}
\caption{Figure 4}
  \includegraphics[alt={},max width=\textwidth]{34de3f03-a275-44fb-88b2-b88038bcec97-23_935_1477_1037_365}
\end{center}
\end{figure}

Figure 5

\begin{center}
\includegraphics[max width=\textwidth, alt={}]{34de3f03-a275-44fb-88b2-b88038bcec97-24_2032_1707_219_153}
\end{center}
\end{enumerate}\end{enumerate}

\hfill \mbox{\textit{AQA D2 2016 Q6 [14]}}