Rounding

Look at the numbers in the question and look at the numbers in the answer choices. Often times, it's not necessary to perform calculations using all the numbers after a decimal. For example, Avogadro's number $6.022 \ x \ 10^{23}$ can often times be rounded to $6.0 \ x \ 10^{23}$ and will facilitate calculations. Practice rounding when you do questions and see how generous you can round numbers.