Add one component to change its percentage: original quantity or amount to add

This type of question, asked 2 times: 8 Aug 2022 Tier II, 6 Mar 2023 Tier II. All 2 questions below, oldest first.

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Question 1
8 Aug 2022 Tier II
67 $A mixture of acid and water contains 20 percent acid . When 10 litres of water is added to the mixture, then the percentage of acid becomes 15 percent. What is the original quantity of mixture?$
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Question 2
6 Mar 2023 Tier II
2 $If 80 litres of milk solution has 60% milk in it, then how much milk should be added to make milk 80% in the solution?$
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Question 1