Ok,
Granted the info in 1 and 2, we still don't know any individual salary and have no reason to believe each individual's salary is equal to last year's average salary. Think about it this way: say the top few salaries increased by x% and the lowest few salaries did not change. The average would then be significantly different than if the lowest few salaries increased by x% and the top few salaries did not change.
I'm not sure exactly where you're from, but you might benefit from one of MR's courses, which can help you develop the math skills you will need for the GMAT.
http://www.manhattanreview.com/cours...urse-list.html
Hope that helped,
JEM