Basic lesson and practice on averages:

http://www.regentsprep.org/Regents/math/ALGEBRA/AD2/indexAD2.htm

There are a couple types of tricky problems related to averages that often show up on the SHSATs:

Finding the average from a frequency table:

Practice 1:

http://www.bbc.co.uk/bitesize/ks3/maths/handling_data/measures_average/revision/7/

Practice 2- don’t worry about the second half of the page, which is more advanced:

https://www.mathsisfun.com/data/mean-frequency-table.html

Missing numbers in averages:

Lesson/ practice 1:

https://www.khanacademy.org/math/cc-sixth-grade-math/cc-6th-data-statistics/cc-6th-mean-median-challenge/v/using-mean-to-find-missing-value

Practice 2:

http://www.softschools.com/math/topics/determining_a_missing_data_value_given_the_mean/