In a town where barter still exists, the following exchange equivalences are available:
- A necklace and a spear are exchanged for a shield.
- A spear is exchanged for a necklace and a knife.
- Two shields are exchanged for three knives.
How many necklaces does a spear cost?
A Spear costs 5 Necklaces.
Shield = Collar + Spear = Collar + (Collar + Knife)
2 Shields = 3 Knives = 4 Necklaces + 2 Knives
1 Knife = 4 Necklaces
Spear = Necklace + Knife = 4 Necklaces + 1 Necklace = 5 Necklaces