Don't tell your high school algebra teacher, but some GRE questions are most efficiently solved by using trial and error, or guessing and checking!
The good news: you barely need any math ability to be able to do this. The bad news: it can feel disorganized and chaotic if you're not used to it.
The below video is my way of solving a question using trial and error. It's taken from the ETS Quantitative Practice Book, pg. 141 #1.
If you liked this video, check out my other free GRE resources like my math strategies page.