[ Evaluation + example ] [ Combinatorial geometry (other).]

What is the minimum number of shots in the game "Battleship" on a 7*7 board needed to definitely hit a four-deck ship (a four-deck ship consists of four cells arranged in a row)?

#
If on a $7 * 7$ board, n non-overlapping four-deck ships can fit, this means that (n-1) shots would not be enough.

## Solution

In the first picture, an example sequence of 12 shots is provided, with which any four-deck ship would be hit. In the second picture, an example of the placement of twelve non-overlapping four-deck ships is shown. Thus, if we have made eleven shots, we would not be able to hit a ship that coincides with one of the twelve shown.

## Answer

12.00