If we walk along the fence from north to south, the distances between its posts are initially the same. From a certain post, the distance decreases to 2.9 meters and remains so until the southern end of the fence. Between the 1st and 16th posts (counted from the north), the distances do not change, and the distance between these two posts is 48 meters. The distance between the 16th and 28th posts is 36 meters.

Which post has different distances from its neighboring posts?

(L. Šimünek)
In the section between the 1st and 16th columns, there are 15 spaces, and according to the problem, their width does not change. Each of them therefore measures $48: 15=3.2$ meters, and is thus 0.3 meters wider than the space at the southern end of the fence.

If all the spaces between the 16th and 28th columns measured the given 2.9 meters, the total distance between these columns would be $12 \cdot 2.9=34.8$ meters. According to the problem, however, this distance is 36 meters, which is 1.2 meters more. This difference can be expressed as $4 \cdot 0.3$ meters, meaning that exactly four spaces in this section between the columns are 0.3 meters longer than the given 2.9 meters. These are the spaces between the 16th and 20th columns.

The sought column, which has different distances from its neighboring columns, is the 20th in order.

Evaluation. 2 points for calculating the size of the northern space $3.2 \mathrm{~m}; 3$ points for the number of spaces of size $3.2 \mathrm{~m}$ between the 16th and 28th columns (or the total number of spaces of size $2.9 \mathrm{~m}$); 1 point for the result.