15. Three friends met - Belov, Serov, and Chernov. Chernov said to the friend dressed in a gray suit: "It's interesting that one of us is wearing a white suit, another a gray one, and the third a black one, but each suit is a color that does not match the surname." What color is each friend's suit?
$\triangle$ Let's take a $4 \times 4$ table. In the left column of the table, we will write the surnames of friends (denoting each by its first letter), and in the top row - the colors of their costumes. According to the condition, Belov does not wear a white costume, Serov does not wear a gray one, and Chernov does not wear a black one. We will place three minuses at the intersection of the corresponding rows and columns of the table (see the table on the left).

|  | w | g | b |
| :---: | :---: | :---: | :---: |
| B | - |  |  |
| G |  | - |  |
| C |  |  | - |


|  | w | g | b |
| :--- | :--- | :--- | :--- |
| B | - | + | - |
| G | - | - | + |
| C | + | - | - |

Next, Chernov does not wear a gray costume, as it is clear from the condition that one of his friends is dressed in gray; we will place a minus in the corresponding cell. Therefore, he can only be wearing a white costume; we will place a plus in the corresponding cell of the table.

Then, Serov does not wear a white costume; hence, he can only be wearing a black costume. Finally, Belov is wearing a gray costume (see the table on the right).

This problem could have been solved without the help of a table by direct enumeration. However, if in a similar problem it is necessary to establish a correspondence between two sets, each of which contains not three, but four, five, or six elements, then solving it with the help of a table is much easier.

Note the following property of the table, which remains valid in similar problems of correspondence between two sets, but only in those cases where these sets contain an equal number of elements: in each row of the table, there is only one plus, and in each column, there is also only one plus. Therefore, if there is a plus in a certain cell of the table, then in the other cells of the same row or column, there can only be a minus.