D2 June 2017 Q6
6.

Figure 1 shows a capacitated, directed network. The number on each arc represents the capacity of the corresponding arc. The numbers in circles represent an initial flow from S to T.
| Scheme | Marks |
|---|---|
| Initial flow = 57 | B1 |
| (1) |
Notes
a1B1: CAO
| Scheme | Marks |
|---|---|
| Value of the cut = 102 | B1 |
| (1) |
Notes
b1B1: CAO
| Scheme | Marks |
|---|---|
![]() | M1 A1 |
| (2) |
Notes
c1M1: Two numbers on each arc and any four numbers correct
c1A1: CAO do give bod since they might well cross these numbers out (in attempting (d))
| Scheme | Marks |
|---|---|
| e.g. SBEGT – 2; SBACT – 5; SBCT – 3; SBEDFGT – 1 e.g. SBCT – 7; SBEGT – 2; SBEDFGT – 1; SBACT – 1 | M1A1A1A1 |
| (4) |
Notes
d1M1: One valid flow augmenting route found and any value stated
d1A1: A second correct flow route and any value stated
d2A1: Three correct flow routes with corresponding correct values
d3A1: CSO flow increased by 11 and no more
| Scheme | Marks |
|---|---|
e.g.![]() | M1 A1 |
| (2) |
Notes
e1M1: Consistent flow pattern \(\geqslant 61\) (check each node). One number only per arc. No unnumbered arcs
e1A1: CAO showing flow of 68
| Scheme | Marks |
|---|---|
| The cut through CT, BT, ET, EG and FG has a value of 68 Value of the flow is 68 so by the max flow – min cut theorem flow is maximal | DB1 DB1 |
| (2) | |
| 12 marks |
Notes
f1DB1: Must have attempted (e) and scored at least M1A1 in (d) – at least one number on all but one arc, and either drawn or stated a cut. Cut may be drawn on any diagram. Note that the cut must separate source (S) from sink (T)
f2DB1: CSO – (e) must be fully correct (showing a correct flow of 68) and a correct cut (either stated or shown on any diagram). Must state the value of 68 in their answer and refer to max flow – min cut theorem – all four words

