There are two warlords: you and your archenemy, with whom you’re competing to conquer castles and collect the most victory points. Each of the 10 castles has its own strategic value for a would-be conqueror. Specifically, the castles are worth 1, 2, 3, … , 9 and 10 victory points. You and your enemy each have 100 soldiers to distribute between any of the 10 castles. Whoever sends more soldiers to a given castle conquers that castle and wins its victory points. (If you each send the same number of troops, you split the points.) Whoever ends up with the most points wins.

But now, you have a spy! You know how many soldiers your archenemy will send to each castle. The bad news, though, is that you no longer have 100 soldiers — your army suffered some losses in a previous battle.

What is the value of the spy?

That is, how many soldiers do you need to have in order to win, no matter the distribution of your opponent’s soldiers? Put another way: What k is the minimum number such that, for any distribution of 100 soldiers in the 10 castles by your opponent, you can distribute k soldiers and win the battle?

(fivethirtyeight.com)

## Solution . . .

As your opponent, knowing you have a spy, how should I allocate my soldiers to make your at-least-28 points costliest?

An “even-cost” strategy allocates soldiers so that for each castle the cost in soldiers-per-point is the same. In this case, since there are a total of 55 points, and since the cost of your winning at any given castle is the number of soldiers I allocate to it plus one, on average over all ten castles the cost is 110/55, or 2 soldiers per point. I can arrange an even-cost situation by sending, to castle number c, 2c-1 soldiers, which uses up all of my soldiers. Your cost of victory will be the 28 points you need times the cost per point of 2 soldiers, for a total of 56 soldiers.

It makes sense that the even-cost strategy is optimal, in the sense of forcing you to spend the greatest number of soldiers, because if I make some castles more expensive than average, other castles have to become below-average bargains, and surely you can then allocate your soldiers so as to favor bargain castles over expensive ones.

## . . . made rigorous

But fans of rigorous proofs will recognize that that’s not one (the word “surely” is always a tip-off that hands are being waved). Let’s try to do better!

Suppose (for reductio ad absurdum) that I have a better strategy than the even-cost one. Because we can assume that I use all 100 soldiers, the sum of the costs to win the ten castles is 110, and given that this strategy is assumed to be better than the even-cost one, the sum of the costs of any set of castles whose numbers sum to 28 or higher is greater than 56. So in particular, 1,8,9,10 cost more than 56 soldiers, as do 1,2,3,4,5,6,7. Since, collectively, 1,8,9,10 cost more soldiers than average (the average cost remaining 2 soldiers per point), the remaining castles, namely 2,3,4,5,6,7, cost fewer than average. Similar reasoning establishes that castles 8,9,10 collectively cost fewer than average.

Castles 2,8,9,10 must together cost more than 56 soldiers, and so castle 2 must cost at least 4 (that is, cost an average or greater number of soldiers per point) to make up for the castles 8,9,10 soldier shortfall by at least 2 soldiers. So 3,4,5,6,7 must collectively cost fewer soldiers than average per point.

Since castles 8,9,10 collectively cost fewer soldiers than average, the same must be true of at least one pair of those three castles (leaving out a castle with the maximum cost per point of the three).

Suppose first that that pair is 9,10. Then any pair from 2,3,4,5,6,7 that sums to 9 (2,7 or 3,6 or 4,5) must cost at least 2 soldiers more than average, because combined with 9,10 they sum to 28 points. But then the six castles from those three pairs together, 2,3,4,5,6,7, cost at least 6 soldiers more than average. Contradiction! So if I really do have a better-than-even strategy, 9,10 can’t be a pair that costs fewer than average soldiers.

Suppose instead that 8,9 cost fewer than average. Then the pairs 4,7 and 5,6 (each of which, added to 8,9, yield 28 points) each cost at least two soldiers more than average, which entails that castle 3 costs at least five soldiers fewer than average—that is, it costs 1 soldier. But then 3,8,9,10 costs fewer than 56 soldiers, despite summing to 30 points. Contradiction!

Finally, suppose 8,10 cost fewer than average soldiers. Then the pairs 3,7 and 6,4 each cost at least two soldiers more than average, and so castle 5 costs at least five soldiers fewer than average—that is, it costs 5 soldiers or fewer. For 5,7,8,10 to cost more than 56 soldiers, castle 7 must cost at least 17 soldiers, or 3 more than average, and for 5,6,8,10 to cost more than 56, castle 6 must also cost at least 17 soldiers, or 5 more than average. Since, together, castles 6 and 7 cost at least 8 more than average, and 3,4,5,6,7 cost fewer than average, 3,4,5 must together cost at least 9 less than average. Then, 3,4,5,8,10, which sums to 30 points, cost at least 10 soldiers fewer than the average cost of 60; that is, those castles can be won with 50 or fewer soldiers. Contradiction!

Therefore our supposition, that there’s a better strategy for me as your opponent than the even-cost one, is false.

There’s glory for you!