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Question
The mean age of 9 children in a class is 15 years. When the teacher’s age is included, the mean age becomes 18 years. Find the teacher’s age.
Sum
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Solution
The mean age of the 9 children is 15 years.
Total age of 9 children = mean × number of children
= 15 × 9
= 135 years
The mean age of 10 people (9 children + 1 teacher) is 18 years.
Total age of 10 people = mean × number of people
= 18 × 10
= 180 years
Teacher’s age = Total age of 10 people − Total age of 9 children
= 180 − 135
= 45 years
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