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There are students in one class whose average age is 19.
If two students leave, the average age of the class is 18.
What is average age of two students who left?
Let = sum of the ages of all students.
Then: . .
Let = sum of the ages of the two students.
Then the other students have a sum of years.
. . Hence: . .
Substitute  into : .
The average age of the two students is: .