Raskina I.V.

Every day the sheep learns the same number of languages. By the evening of his birthday, he knew 1000 languages. By the evening of the first day of the same month, he knew 820 languages, and by the last day of the month - 1100 languages. When is the sheep's birthday?
In the month when the sheep's birthday occurs, not counting the first day, the sheep learned $1100-820=280$ languages. A month can have 28, 29, 30, or 31 days. Therefore, the month minus one day has 27, 28, 29, or 30 days. Since the sheep learns the same number of languages each day, 280 must be divisible by the number of days without a remainder. The only number that satisfies this condition is 28, so the sheep learns $280: 28=10$ languages per day. From the second day of the month until the birthday, the sheep learned $1000-820=180$ languages. Therefore, 18 days have passed, which means the sheep's birthday is on February 19.

## Answer

February 19.

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