1. Tells us that x is the median and can= 8,9,10. We're told that the median=mean, so we can plug in each value (8,9,10) into the rest of the set to find the average. The number that makes the mean=median is the correct answer. If there's more than one correct answer the statement is insufficient:

Again, we know that x is the median, we're trying to find the value that makes the average of the set equal to the median

using x= 8:
(2+7+8+11+16)/5 does not = 8

x=9
(2+7+9+11+16)/5 does = 9

x= 10
(2+7+10+11+16)/5 does not=10

Because 9 is the only value that works, the statement is sufficient

2. Says the same thing as the Statement 1 (x=8,9 or 10) so we know it's sufficient.

Thus, D is the answer.

Hope that helps

Agree with mim3. (D) it is.

(1) This tells you that x is the mean. Whenever the the mean=median, the numbers are normally distributed. This tells you that x has to be equally spaced between 7 and 11, which means that it's 9.
(2) See above

would this be a 600/650/700/700+ level question?