One of the reasons you may be considering the GRE is because it has a calculator. That of course is of course good news! However, the calculator that they provide is pretty terrible and that is the bad news. So, even though you have the calculator available those seeking to score above a 160 on the quantitative section may benefit from eschewing the calculator for simple arithmetic in the interest of time. Remember, you only have one minute and 45 seconds per question on these fast paced GRE quantitative sections!

Manual Calculation Tips
First, it is simply a fact of the exam, and honestly, life that memorizing your times and division tables up to at least 12 squared, and preferably 15 squared will help you make mental math more efficient. So, hop over to math-aids.com and begin practicing those multiplication and division times tables!

Beyond those memorized steps though, how can you best process mental arithmetic efficiently? It comes down to biology! Have you ever considered why humans use a decimal, or multiple of tens, based counting system? It's because we (with some limited exceptions) have ten fingers and ten toes! If evolution had dictated that we had eight fingers per hand, we'd be basing all of our math off of multiples of 8 and 16. For this reason, you will want to break all of your mental arithmetic into multiples or factors of 2, 5, 10, 100, or 1/2 to make the calculation go quickly in your head.

Let's consider the example of 24 + 82. First, move the 2 from 82 to 24 to make the operation 26 + 80. Now, add the 20 to the 80 to get 100 and finally add the 6 to determine that 24 + 82 = 106 in a certain and efficient fashion! Of course, you can always use the calculator when in doubt, or dealing with three or more digits or complex decimals, but this sort of mental mathematics will save you the time of opening and working with that terrible GRE calculator.

Manipulating Fractions
Now, let's talk about the most difficult number format for the terrible calculator - fractions. Many a GRE test taker eschews fractions for decimals any time they are encountered on the exam. This is a huge and unnecessary waste of time! Just remember a few basic tenets for fractions and you'll be in great shape.

1. Memorize Common Conversions - 1/2 = 0.5 | 3/4 = 0.75 | This is just a very short introductory list, but you should probably memorize every fraction to decimal conversion from 1/2 to 1/10 except for 1/7 and be able to figure out every fraction for each of those denominators < 1, too. For instance, 3/8 = 0.25 + 0.125 = 0.375!
   
2. Use Common Denominators for Addition & Subtraction - Remember that you must multiply your numerator and denominator by 1 to produce a common denominator before adding or subtracting the numerators only, while keeping the now common denominator. For instance, 3/8 + 2/5 = 15/40 + 16/40 = 31/40!
   
3. Divide First to Take Easy Fractions of Wholes - One of the most common requirements with fractions is to take a fraction of a whole and the most efficient way is to divide first by the denominator rather than multiplying up and then dividing back down. For instance, taking 3/15 of 75 is much easier when you divide 75 by 15 first to produce 5 and then multiply that result by 3 to determine that 9 is 3/15 of 75!

Taking and Translating Easy Percentages
Yes, percentages can be completed really easily with the terrible calculator. In fact, percentages are one of the computations it does reasonably well. But you too can complete percentage calculations easily, at least simple ones, and again avoid wasting precious seconds engaging the clunky GRE interface.

Much like our biology dictates all arithmetic, it basically dictates percentages too, since all percents are calculated out of 100. In fact, these percentages were developed so that merchants before calculators could discount wares quickly, so if a wool merchant in the dark ages could do it, so can you!!

Let's go with the example and say that you normally sell a bolt of wool for $50, but someone wants you to take 20% off in negotiations. Easy enough! We know that 20% is simply twice 10% and 10% of $50 is $5, so 20% off would be 50 - 2(5) = $40.

Here are the steps for other percent questions too:

1. Carefully Process the Information - Make sure that you don't rush percentage problems, especially word problems. If you confuse "percent of" with "percent off" you'll get a different answer. Same with percent increase versus percent decrease. Read carefully to make sure you're solving for the intended value.
   
2. Take 10% or 1% of the Value by Shifting the Decimal - This is why those centuries old merchants used percentages! It's so easy to take 10% by shifting the decimal left one place or 1% by shifting that same decimal two places.
   
3. Multiply or Divide Proportionally to Reach Exactly Sought Percent - Usually, it's easier to go up than down, but either way as long as you are careful you will reach the correct value and without clicking for the calculator. For instance, 7% of 400 just requires taking 1% or 4 and then multiplying that value by 7 to determine that 4 x 7 = 28 is 7% of 400!

With these simple arithmetic tips you'll easily halve the amount of time you reach for the terrible calculator with your GRE cursor, giving yourself more time to logically work harder problems as you reach for a 170. However, remember that until you are 100% comfortable with these mental math tactics, it's better to use the terrible calculator as crutch than to make an unforced error!