6.4. Petya and Masha made apple juice. In total, they got 10 liters. They poured it into two jugs. But it turned out that it was hard for Masha to carry her jug, so she poured some of the juice into Petya's jug. As a result, Petya ended up with three times more, and Masha ended up with three times less. How many liters did Petya have to carry?
Answer: 7.5 liters.

Solution: Let's assume that before Masha pours the juice to Petya, she pours it into a separate bottle. According to the problem, if this juice is added to Petya's, he will have three times more juice (and Masha will have three times less). This means that currently, Masha and Petya have the same amount of juice, and in the separate bottle, there is twice as much juice as each of them has. This means that the bottle currently contains half of all the juice they have, which is 5 liters, and Petya and Masha each have 2.5 liters. Therefore, in the end, Petya will have $2.5+5=7.5$ liters.