Question
At first, Barrel P and Barrel Q contained 0.2 ℓ and 4.2 ℓ of water respectively. Some water from Barrel Q was poured into Barrel P with no water spilling over. In the end, there was as much water in Barrel P as in Barrel Q. How much water was poured from Barrel Q to Barrel P?
3 m

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At first, Barrel P and Barrel Q contained 0.2 ℓ and 4.2 ℓ of water respectively. Some water from Barrel Q was poured into Barrel P with no water spilling over. In the end, there was as much water in Barrel P as in Barrel Q. How much water was poured from Barrel Q to Barrel P?