variables in equations

Yes, definitely, this is a major part of the exam.

This is going to sound obvious, but the best advice is to practice. Plugging in and manipulating equations are both very formulaic - it just takes a lot of repitition and analysis to "get it". One thought: if you're having serious trouble with basic math, consider purchasing a math workbook - not a GMAT prep book, but just a math book with lots and lots of basic equations and problems to solve. Then, after you've practiced "non GMAT" math for awhile, go back and try to do GMAT problems. Another though: if you have the time, try taking a basic math class at a local college. I've had a couple students do this, with great results.

Good luck!

thanks do not have a lot of time maybe i will get a high school math tudor?

Yes, a tutor is a good idea. Remember that you don't necessarily need to work on GMAT problems - practice in basic math and algebra will provide you a solid foundation, then you can add on the GMAT problems later.