b. Have node pairs such as T1, T1' for all twelve targets on
both days. For day 1, Identify the flow on the target arcs be
t_{j11 }and t_{j21}, where j is the target index,
1 refers to the arc with cost d_{j} and 2 refers
to the arc with cost 0.75d_{j}. Similarly define
t_{j12} and t_{j22} for the second day.
Define the binary variables, w_{j} = 1 if target is
hit on the first day and 0 if it is hit on the second day, for
j = 1 to 12.
Add side constraints: t_{j11} + t_{j21} ¾ 10w_{j}
and t_{j12} + t_{j22} ¾ 10(1  w_{j}),
for each target j. This forces the flows on a target in day
1 to be 0 unless the target is selected. It forces the flows
on a target in day 2 to be zero unless it is not selected for
day 1.
This is a mixed integer programming model.
