On Grockit, you can create a "Custom Game" with only those skill tags to practice those types of questions only. Grockit offers a limited number of questions for a free trial membership, so you might want to check that out!
Purple Math is a great site to review basic Math concepts but it's not GMAT-specific:
https://www.purplemath.com/modules/index.htm
You can also search Beat the GMAT's articles now, for everything pertaining to certain skill tags. Here's what my search for "Ratios" turned up:
https://www.beatthegmat.com/mba/category ... ionsratios
Good luck!