to make sure that you are really using the correct solution during the exam. Because if during your quick trial and error you do not come up with 1/4 it only means that your solution is wrong and that you
cannot use the same in solving similar problems in your exam. 

      With division of fractions you should memorize, really memorize that 1/2 divided by 1/2 should equal 1. This memorization of answers will only give you the confidence that you're indeed using the correct procedure in answering the questions during your exam. I will not teach you on how to divide or add fractions because I assume that you already studied and learned these.

      With Subtractions you should memorize that 1/2 - 1/2 = 0 for similar fraction and 1/2 - 1/4 should equal 1/4 for dissimilar. You could also memorize answers for mixed fractions that you can use for quick trial and error in your exams.  You can also use this math strategy in other kinds of