A tanker of milk was delivered to the store. The seller has balance scales without weights (fляги can be placed on the pans of the scales), and three identical flasks, two of which are empty, and the third one contains 1 liter of milk. How can you pour exactly 85 liters of milk into one flask, making no more than eight weighings?

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First, let's place two empty flasks on one scale pan and on the other pan - a flask containing 1 liter of milk, and pour milk into one of the empty flasks until the scales balance. Then, this flask will contain (1 - a) liters of milk, where $a$ is the number of liters of milk that balances one empty flask. Next, let's place on one scale pan the flask with one liter of milk and the flask with (1-a) liters of milk, and on the other pan, place an empty flask and pour milk into it until the scales balance. Then, this flask will contain two liters of milk. Pour the contents of this flask into the flask that had one liter. Thus, we will have one flask with 3 liters of milk and another with (1-a) liters of milk. Similarly, having flasks with (1-a) liters of milk and n liters of milk, we can obtain flasks with (1-a) liters of milk and (2n+1) liters of milk, and if we place the flask with (1-a) liters of milk on the other pan, we can obtain flasks with (1-a) liters and (2n-1) liters of milk. Acting in this way, we sequentially obtain 3, 5, 11, 21, 43, 85 liters of milk. Thus, we can pour exactly 85 liters of milk into one flask in seven weighings.