8.1. In the wagon, several kilograms of apple jam were loaded, of which $20 \%$ was good and $80 \%$ was bad. Every day, half of the existing bad jam rotted, and it was thrown away. After several days, it turned out that $20 \%$ of the jam in the wagon was bad and $80 \%$ was good. How many days have passed since the loading?
Answer: 4 days.

Solution 1: Let the initial total amount be $x$, and the final amount be $y$ kilograms of jam. Then, since the amount of good jam did not change, $0.2 x = 0.8 y$, which means $x = 4 y$. Therefore, initially, the amount of bad jam was $0.8 x = 3.2 y$, and it became $0.2 y$, meaning the mass of bad jam decreased by a factor of 16. Given that the amount of bad jam halved each day, it is clear that 4 days have passed.

Solution 2: Initially, there were 100 arbitrary units of jam: 20 good and 80 bad. In the end, the amount of bad jam became $20 \%$, and the amount of good jam became $80 \%$ of the new quantity, meaning the amount of good jam increased by a factor of 4. Therefore, the new amount of bad jam is 5 arbitrary units. Since it decreased by a factor of 2 each time and ultimately decreased by a factor of 16, 4 days have passed.

Criteria: Only the answer, answer with verification - 1 point. In a solution similar to the second one, if abstract units of measurement are not used but specific ones are used without any explanation that the units of measurement are not important - 1 point.