Suppose $n$ is a number such that the mean (average) of the list of numbers $2,9,4, n, 2 n$ is equal to 6 . What is the value of $n$ ?
(A) 9
(B) 12
(C) 10
(D) 5
(E) 6
The given list, $2,9,4, n, 2 n$ contains 5 numbers.

The average of these 5 numbers is 6 , and so the sum of the 5 numbers is $5 \times 6=30$.

That is, $2+9+4+n+2 n=30$ or $15+3 n=30$, and so $3 n=15$ or $n=5$.

ANSWER: (D)