## 36. Sacks of Flour

Two trucks are transporting identical sacks of flour from France to Spain. The first one carries 118 sacks, while the second one carries only 40. Since the drivers of these trucks do not have enough pesetas to pay the customs duty, the first one leaves 10 sacks for the customs officials, as a result of which he only has to pay 800 pesetas. The second one does the same, but he had to unload only 4 sacks, and the customs official even paid him 800 pesetas.

How much does each sack of flour cost, given that in these calculations the customs officials took exactly as much flour as was required to fully pay the customs duty?
36. Let $x$ be the cost of one sack of flour, and $y$ be the duty charged per sack.

If the first driver left 10 sacks at the customs, he should pay duty only for 108 sacks. Similarly, the second driver should pay duty only for 36 sacks. As a result, we get the following system of equations:

$$
\begin{aligned}
10 x+800 & =108 y, \\
4 x-800 & =36 y .
\end{aligned}
$$

Multiplying the second equation by 3 and subtracting the first equation from it, we get

$$
2 x-3200=0
$$

Therefore, $x=1600$.

Each sack of flour costs 1600 pesetas.