GMAT Math

Hello,
I seem to have trouble answering Average questions, especially in DS, are there any rules/principles that you are guys are aware of regarding finding the average from a list of numbers? (when the complete set of numbers is not given).

For instance, I think I read somewhere that in a list of consecutive odd integers, the median is the average. Is that true?
Tidbits like this will come in handy.

Thanks