3. The store received 206 liters of milk in barrels of 10 and 17 liters. How many barrels of each type were there?
# 3. Answer: 7 and 8.

The total volume will be $10 x+17 y$ and equals 206 liters. Let's look at the last digit of the sum $10 x+17 y$. Clearly, it depends only on y, because the number $10 x$ always ends in zero. Therefore, we need to find such a y that $17 y=$ ...6 and $17 y \leq 206$. Only y $=8$ fits, as $17 \times 8=136$, so $x=$ $(206-136) / 10=7$.