13. The total corn harvest in centners collected from a certain plot of land is expressed by a four-digit number using the digits 0, 2, 3, and 5. When the average yield per hectare was calculated, it turned out to be the same number of centners as the area of the plot in hectares. Determine the total corn harvest.
13. Since the total harvest is equal to the product of the yield and the area of the plot, the four-digit number representing the total harvest is the square of some two-digit number. This square cannot end in the digits 0, 2, or 3. Therefore, the units digit of this four-digit number can only be 5. The tens digit can only be 2, then the hundreds digit is 0, and the thousands digit is 3. Thus, the total harvest is 3025 tons, and the yield per hectare is 55 tons, since $55^{2}=3025$.