7-5. In a row, there are 1000 toy bears. The bears can be of three colors: white, brown, and black. Among any three consecutive bears, there is a toy of each color. Iskander is trying to guess the colors of the bears. He made five guesses:

- The 2nd bear from the left is white;
- The 20th bear from the left is brown;
- The 400th bear from the left is black;
- The 600th bear from the left is brown;
- The 800th bear from the left is white.

It turned out that exactly one of his guesses is incorrect. What could be the number of the bear whose color Iskander did NOT guess correctly? Select all possible answers.
Answer: 20.

Solution: Since among any three consecutive bears there is a bear of each color, the numbers of all bears of a certain color have the same remainder when divided by 3. Indeed, let's look at the bears with numbers $n, n+1$, and $n+2$, as well as with numbers $n+1, n+2$, and $n+3$. In both cases, there will be bears of all three colors only if the bears with numbers $n$ and $n+3$ are of the same color.

At least one of the guesses "the 20th bear is brown" and "the 600th bear is brown" is incorrect: since the numbers 200 and 600 give different remainders when divided by 3. Also, at least one of the guesses "the 2nd bear is white" and "the 20th bear is brown" is incorrect: since the numbers 2 and 20 give the same remainder when divided by 3, therefore, the bears with these numbers must be of the same color. Thus, the guess "the 20th bear is brown" is definitely incorrect.

Comment: As follows from the solution, the situation described in the problem is possible: all bears with numbers giving a remainder of 1 when divided by 3 must be black, a remainder of 2 - white, and a remainder of 0 - brown. This is the only possible situation.