Average of a pair and of the pair plus a third: find the third or a new average

This type of question, asked 2 times: 17 Jul 2023 S1, 21 Sep 2025 S3. All 2 questions below, oldest first.

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Question 1
17 Jul 2023 S1
52 $There are 3 members in a family. The average age of the father and mother is 40 years. The average age of the father, mother and daughter is 30 years. Find the age (in years) of the daughter.$
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Question 2
21 Sep 2025 S3
66 $The average of x and y is 30. The average of x, y, and z is 40. What is the average of x, y, and 2z?$
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