507. Fresh mushrooms contain $90\%$ water, while dried ones contain $12\%$ water. How many kilograms of dried mushrooms can be obtained from 44 kg of fresh mushrooms?
$\triangle$ According to the condition, 44 kg of fresh mushrooms contain $44 \cdot 0.9 = 39.6$ kg of water, which means there is $44 - 39.6 = 4.4$ kg of dry matter.

Let's denote the mass of dried mushrooms that can be obtained from 44 kg of fresh mushrooms by $x$ kg. These $x$ kg consist of $0.12 x$ kg of water and $0.88 x$ kg of dry matter.

Since the mass of the dry matter is the same in both fresh and dried mushrooms,

$$
0.88 x = 4.4
$$

Therefore, $x = \frac{4.4}{0.88} = 5$.

This problem could also be solved without algebra - arithmetically.

Answer: 5 kg.