3) What is the mean (arithmetic) of the numbers 1, 2, 3, . ., 1999, 2000?
(A) 999
(B) 999.5
(C) 1000
(D) 1000.5
(E) 1001.
3) The answer is $(D)$

Just pair the first number with the last, the second with the penultimate, and so on, until you get 1000 pairs. Since the sum of two numbers in a pair is 2001, we will have that the sum of all the numbers is $S=2001 \times 1000$. Therefore, the arithmetic mean is $S / 2000=2001 / 2=$ 1000.5.