GMAT DS
Quantitative 4 – When to avoid the Venn
Quantitative 3 – Looking from the other side
DS – Weighted Averages
For more detail on weighted averages, see the following previous posts:
https://crackthecat.wordpress.com/2013/01/21/weighted-averages-1/
https://crackthecat.wordpress.com/2013/01/22/weighted-averages-2/