Help with technique for quick multiplication of numbers

Question

Hi guys,

Anyone could refer any material to quickly multiply large numbers?

I applied the doubling and halving technique and it works pretty well for some non-prime numbers. (E.g. 75 * 500 = 150 * 250 , so on)

However, I cannot save time by using it in the multiplication of primes and non-primes (63 * 17) or long numbers (2563 * 8584).

In sum, is there useful tips to apply for numbers between 20 - 99 (E.g. 17*19) and above the 100's.

Thanks a lot,

GMATPill · Answer

Hi felippemed,

What you can do is round the numbers. So 17 * 19 ---is close to 20 * 20 --- but a little but less. 20 * 20 = 400 -- so that means the original 17*19 should be a little bit less.

If given multiple choice -- you should avoid the answer choices that are 400 and above and choose the one that is slightly below 400.

The same technique can be used for other numbers.

felippemed · Answer

Well Noted! Excellent!!!

I gonna include your tip in my annotations. Thank you very much for that...

However, I was looking for any "rule of thumb" for larger numbers. I am aware of one for numbers between 11 and 19, as follow:

E.g. 17 * 19 = (17+9)*(19-9)+(9*7) = 323

EMPOWERgmatRichC · Answer

Hi felippemed,

The GMAT won't require that you multiply "big" numbers together in the Quant section, so if you think that you must do that to answer a given GMAT Quant question, then there is almost certainly an easier/faster way to get to the solution.

Have you come across any prompts that made you think that this type of math was required? If you post them in the respective Quant Forum(s), you'll almost certainly receive explanations and advice that will show you how you can avoid doing that type of math.

GMAT assassins aren't born, they're made,
Rich

felippemed · Answer

Hi EMPOWERgmatRichC , Thank you for your feedback. Since this section is called "General GMAT Questions and Strategies," I thought it was the right place. Next time I will place it in the Quant Section. My bad. So, there is a question in the test code 25, section 7, number 1, that exemplify the need. Probably, as you said, a better technique may be more efficient and available, but since I got this notation from the very first page of the all-u-need-for-gmat-quant-90489.html#p689013 I thought it would be useful to understand some parts that I could not - which by the way is the third most downloaded quant notation in the Forum with >200 kudos.

EMPOWERgmatRichC · Answer

Hi felippemed,

The fact that you posted in this specific Forum is fine - I was just pointing out that if you'd like to post a specific practice Quant question, then you should do so in the PS or DS Forums so that other Experts and Users can take a look at it (and offer advice, solutions, etc.). In that same way, if you want to research a specific question, you can search for it here (or Google it) - there's a pretty good chance that it's already been posted and discussed.

GMAT assassins aren't born, they're made,
Rich

HanoiGMATtutor · Answer

If you have to resort to calculate anything resembling 123*789, you probably did not approach the problem efficiently. Multiplication of numbers below 20 are relatively simple; you can learn to perform these calculations in your head. 14*19 = 19*14 = 19*10 + 19*4 = 190 + 76 = 266 15*19 = (17-2)*(17+2) = 17^2 - 2^2 = 289 - 4 = 285 Memorizing the squares of numbers below 20 is very helpful. :wink:

dabral · Answer

I am no longer active on this forum.

felippemed · Answer

Thank you  HanoiGMATtutor !

it is very helpful your tip! Starting to use it straight away

felippemed · Answer

Very kind of you  dabral

I am putting together all your tips to improve my calculation.

Just recognized that is somehow a way GMAT test one of my skills!

Thank you very much

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Help with technique for quick multiplication of numbers

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