Question 71
One of the cells of $20 \times 20$ torus contains a buried treasure. Today, in order to find the treasure we select several rectangles $1 \times 4$ or $4 \times 1$ on this torus and ask the sapper to investigate them by a mine detector. The results of all investigations will be known tomorrow, for each rectangle the sapper will tell us if the treasure is in this rectangle. What is the minimal number of rectangles we should select in order to find the cell that contains the treasure?