Combined average known: find a group's size or the ratio of group sizes

This type of question, asked 4 times: 20 Jul 2023 S1, 24 Jul 2023 S1, 9 Sep 2024 S1, 15 Sep 2025 S3. All 4 questions below, oldest first.

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Question 1
20 Jul 2023 S1
64 $The average age of a group of friends is 27 years. If 4 new friends whose average age is 25 years join them, the average age of the entire group becomes 26 years. How many people were there in the group initially?$
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Question 2
24 Jul 2023 S1
59 $During a school excursion each student of junior school was charged ₹325 and each student of senior school was charged ₹400. If there were 80 students from junior school and the combined average amount charged per student was ₹352, then how many students from senior school went for the excursion?$
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Question 3
9 Sep 2024 S1
69 $100$
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Question 4
15 Sep 2025 S3
56 $The overall average salary for all employees in an office is ₹16,000. Officers earn an average salary of ₹28,000, while non-officers make an average of ₹10,000. If there are 15 officers, determine the number of non-officers.$
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Question 1