Problem of the Week 1224
A perfectly camouflaged tank sits on one square of a 41 × 41 square chessboard-like field.
Two shots are necessary to destroy the tank. After the first hit, and only then, the tank moves to an adjacent square in the horizontal or vertical direction, still invisible.
If a shot strikes the content of the single square it is aimed at, what is the minimal number of shots needed to guarantee the tank's destruction?
Source: A Russian contest problem that appeared in the Mitteilungen der DMV and was relayed to me by Bob Burckel. I do not know the answer.