13.013. In January, the plant fulfilled $105\%$ of the monthly production plan, and in February, it produced $4\%$ more than in January. By what percentage did the plant exceed the two-month production plan?
Solution.

Let $x / 2$ be the monthly plan of the plant; $x$ be the two-month plan. In January, the plant exceeded the plan by $0.05 \cdot x / 2$; in February by $0.04 \cdot 0.05 \cdot x / 2$. Over two months, the plant exceeded the plan by $0.05 \frac{x}{2} + 0.04 \cdot 0.05 \frac{x}{2} = 0.071 x$. Therefore, the plant exceeded the two-month production plan by $7.1\%$.

Answer: $7.1\%$.