Answer
3
120
8
5
Work Step by Step
To the guard, the chocolate bars and bread have the same value so 3 chocolate bars can be exchanged for 3 loaves of bread.
In camp, each loaf of bread is worth 40 cigarettes, so the 3 loaves can be exchanged for 120 cigarettes.
Back in camp, the 120 cigarettes can be exchanged for 8 chocolate bars.
Since the POW started out with 3 chocolate bars, the net gain is 5 chocolate bars.
