Problem 1. After $\frac{1}{4}$ of the gasoline was poured out from the barrel, and then another $10\%$ of the total amount in the barrel, 26 liters of gasoline remained. How many liters of gasoline were there in the barrel at the beginning?
Solution. Let there be $x$ liters of gasoline at the beginning. Then

$$
x-\frac{1}{4} x-\frac{10}{100} x=26
$$

From this, $x=40$.