GMAT: Problem Solving
Select AN answer, don't compute THE answer: Because GMAT supplies five possible answer choices to each question, the task is not to calculate or derive an answer, but to select one  a subtly different task.
So in addition to the traditional Analytical "calculate the answer" approach indoctrinated into us at school, it is often quicker and more effective to explore two other approaches:

Numerical (aka substitution, backsolving, "plugging in the answers"), or

Logical ("which answers can I eliminate without calculation ?")
Workings don't matter! Too many candidates adopt the dutiful, exhaustive, "show your workings" approach. GMAT doesn't credit your workings. So, exercise some operational flexibility and be aware that there exist three valid avenues to solving Quant questions. Similarly, Elimination is often the best way to solve Verbal questions.
Do not overcalculate: If the question contains the word "approximately" or offers answer choices are widely dispersed in value, GMAT is signalling you that you should save some time by making approximations. Conversely, if the values of the answer choices are close together, this is an indication that GMAT expects you derive the answer precisely.