Statistically-challenged? GMAT tests only a few stats concepts: mean, median, mode, range, standard deviation, min/max, and weighted averages. You probably have seen some of these already!
This sample video question will introduce you to the concept of “average” or the “arithmetic mean.” This is the most highly-tested Statistics concept on the GMAT. The takeaway formula is Average = sum of terms / # of terms.
This video explains how to find weighted averages, which is different from the arithmetic mean (though frequently confused with it). Find the SUM of the each group first, then add the SUMS for the numerator, and combine the total number of elements for the denominator to find the weighted average.
Check out video explanations for other stats concepts such as median, mode, mean, and standard deviation on this Learnboard!