The GMAT And No Calculator

Fact: You cannot bring your own calculator to the GMAT

Fact: the Integrated Reasoning section will have an onscreen calculator.

Fact: the Quantitative section is 100% calculator-free

The thought of doing math without a calculator strikes fear into the hearts of many, but have courage!  First of all, the fact that the Quantitative section is 100% calculator-free places enormous restraints on the authors of the test: they can only ask question that can be answered in a reasonably efficient manner without a calculator.  Furthermore, with a few key tricks, you can enormously improve your sans-calculator performance.

Practice Mental Math Daily
There’s no substitute for knowing your basic arithmetic.  That’s a “muscle” you needs to build up to champion form by the time of the GMAT, so if those skills are rusty now, you need to train like an athlete between now and the GMAT.
This means: do mental math every day.  Of course, of course, do all your GMAT math practice questions without a calculator.  But beyond that, find little ways to do arithmetic every day.  You’re in line at the grocery store: what’s the total of your order to the nearest dime?   When you notice any change in price, figure out the percentage change in your head.  Do these in your head, and then check with a calculator, or have a friend check.  It will be hard at first: be doggedly determined in your practice, and you will get faster.

Estimation
Never perform an exact calculation when estimation will do.  If the answer choices are widely spaced, then that’s essentially an engraved invitation to estimate.  For example, if you see 48.9 x 8.11, you know that’s going to be very close to 50 x 8 = 400, and in all likelihood, that will be enough to isolate an answer on a Problem Solving question.

Cancel BEFORE you multiply
This is a gigantic timesaver, and nobody seems to talk about this. Suppose, in the course of a GMAT math problem, you have to solve the following proportion for x:

96/x = 24/15

Of course, the first step will be to cross-multiply:

96*15 = 24x

At this point in the problem, an unstrategic GMAT taker would make the disastrously bad move of finding the product of 96*15.  Under no circumstances should you multiply two two-digit numbers unless you are utterly bereft of alternatives.
The strategic next step would be to divide by 24, to isolate x. Dividing by 24, we have

x = (96*15)/24

and at this point, we are in a perfect configuration to cancel.  We can cancel a factor of 3 from both 15 and 24, leaving 5 and 8 respectively.

x = (96*5)/8

Now, we notice that 8 divides evenly into 96 —- 96/8 = 12

x = 12*5 = 60

Now what is easier: calculating 96*15, then dividing that four-digit number by 24, or simply multiplying 12*5?  Internalize this habit, and you will increase both your accuracy and your problem-solving efficiency.

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