You have five pieces of chain, each consisting of three links. You want to make one long chain of these five pieces. Breaking open a link costs 1 euro, and welding an open link costs 3 euros.
Is it possible to make one long chain of the five pieces, if you have just 15 euros?
Yes, this is possible. First, break open all three links of one of the pieces of chain. This costs 3×1 = 3 euros. Then join the remaining four pieces of chain with the three open links. Welding these links costs 3×3 = 9 euros. The total costs are 12 euros.
