Originally Posted by pan6467

shouldn't 1.D be 1998 for the average? The set is increasing by 4 thus we have this:

4*999=3996/2=1998

It's not in the set but you are looking for the average off an odd # based on an even set therefore you should get an odd # (2 in this case being odd).

4*1000=4000/2=2000

B and C should be 999

2*999=1998/2=999

(1+2)+(2*998)/2=999 (because you are still using 2*999, however you need are starting at 1 and going up with odd #'s. The way you have 998 and not 999*2 is because the 1+2 counts as the 999th #.