13.328. A fishing brigade planned to catch 1800 tons of fish within a certain period. For $1 / 3$ of this period, there was a storm, as a result of which the planned daily target was underachieved by 20 tons. However, in the remaining days, the brigade managed to catch 20 tons more than the daily norm, and the planned target was completed one day ahead of schedule. How many tons of fish were planned to be caught daily?
## Solution.

Let $x$ be the planned period, and $y$ be the number of centners per day according to the plan. According to the conditions, we have the system $\left\{\begin{array}{l}x y=1800, \\ \frac{x}{3}(y-20)+\left(\frac{2 x}{3}-1\right)(y+20)=1800,\end{array}\right.$ from which $y=100$.

Answer: 100 centners.