In the list $7,9,10,11,18$, which number is the average (mean) of the other four numbers?
(A) 9
(B) 18
(C) 7
(D) 11
(E) 10
The average of the numbers $7,9,10,11$ is $\frac{7+9+10+11}{4}=\frac{37}{4}=9.25$, which is not equal to 18 , which is the fifth number.

The average of the numbers $7,9,10,18$ is $\frac{7+9+10+18}{4}=\frac{44}{4}=11$, which is equal to 11 , the remaining fifth number.

We can check that the averages of the remaining three combinations of four numbers is not equal to the fifth number.

Therefore, the answer is 11.

(We note that in fact the average of the original five numbers is $\frac{7+9+10+11+18}{5}=\frac{55}{5}=11$, and when we remove a number that is the average of a set, the average does not change. Can you see why?)

ANSWER: (D)