There are 26 students in a class and one of them who is 14 years old leaves the class and his place is filled by a newcomer. If the average age thereby is lowered by one month, what is the age of the newcomer?

Using the replacement method, the newcomer's age equals the leaving person's age minus the total decrease in the class's age sum. The average decreases by one month for all 26 students, making the total decrease 26 months. Subtracting 26 months (which is 2 years and 2 months) from 14 years gives an age of 11 years and 10 months. The age of the newcomer is 11 years and 10 months.

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