The average age of a class of 10 students is 15.

AAPL wrote:The average age of a class of 10 students is 15. A year later, one more student joins in. The average age of the class becomes 16 years. What is the age of the newest member of the class?

A. 26 years
B. 22 years
C. 20 years
D. 18 years
E. 16 years

Alternate approach:

Since the average age of the 10 original students = 15, the sum of the original 10 ages = (count)(average) = 10*15 = 150.
One year later, each of the 10 students is 1 year older, increasing the sum of the ages by a total of 10 years:
150+10 = 160.
After the addition of a new student, the average age for the resulting 11 students =16, yielding the following new sum:
(count)(average) = 11*16 = 176.
The age of the newest student must be equal to the difference between the two blue sums above:
176 - 160 = 16.

The correct answer is E.

AAPL wrote:The average age of a class of 10 students is 15. A year later, one more student joins in. The average age of the class becomes 16 years. What is the age of the newest member of the class?

A. 26 years
B. 22 years
C. 20 years
D. 18 years
E. 16 years

If the average age of a class of 10 students is 15 years old now, then a year later, the average age of the class will be 16 years old. Since the average age of the class is also 16 years old when a new student joins in, his or her age must be 16 years old also.

Answer: E