I can't seem to get all the reading comprehension questions completed within the time limit.
It's often faster in the reading section to start with the questions instead of reading the whole passage first. Skip any questions that are broad & apply generally to the whole passage (usually the first 1-2 in a section), and start right with the 1st specific question. It's also easier to eliminate wrong answer choices than to immediately find the correct one, so start there and work backwards. One last tip is to identify the most time-consuming questions and skip those until your score comes up to where you'd like it to be; then you can go back and add those in.
How can I tell whether I have written a good thesis or not?
Your thesis needs to match the writing assignment or project you're working on (sometimes different writing forms have different types of theses), and then the most important thing is that you express your opinion or idea that you're going to prove. As your writing becomes more sophisticated and you have some practice with solid thesis writing, you can improve your thesis so that it also contains a short preview of the arguments you're going to make to support it.
I can never remember how to simplify fractions with exponents; x^5 / x^2 . . . do you add or subtract the exponents?
Well, the shortcut is that you subtract, but if you ever forget on a test, you can simply write out the fraction in its long form, and cancel out the top & bottom (numerator & denominator, if you like fancy terms). So, xxxxx / xx = xxx, or x^3.