[ Arithmetic. Mental calculation, etc.]

In the basket, there are 30 russulas and boletus. Among any 12 mushrooms, there is at least one russula, and among any 20 mushrooms, there is at least one boletus. How many russulas and how many boletus are in the basket?

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Since out of 12 mushrooms there is at least one russula, then there are no more than 11 boletus, and out of any 20 mushrooms there is at least one boletus, which means there are no more than 19 russulas. Since there are 30 mushrooms in total, then there are 11 boletus and 19 russulas.