1999 AMC 8 Problems/Problem 13

The average age of the 40 members of a computer science camp is 17 years. There are 20 girls, 15 boys, and 5 adults. If the average age of the girls is 15 and the average age of the boys is 16, what is the average age of the adults? $\text{(A)}\ 26\qquad\text{(B)}\ 27\qquad\text{(C)}\ 28\qquad\text{(D)}\ 29\qquad\text{(E)}\ 30$

The answer is (C). First, find the total amount of the girl's ages and add it to the total amount of the boy's ages. It equals 540. You have to see what can give you 17 when you divide by 40. So, you do $17\cdot40=680$ Then you do $680-540=140$ The 5 adult's ages should add up to 140. You then do $140\div5=24$ . So, the answer is (C).

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1999 AMC 8 Problems/Problem 13