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Problem Solving — algebra and arithmetic (GMAT Focus Edition)
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This is related to the GMAT, I swear!

by Brian@VeritasPrep » Thu Sep 09, 2010 9:51 am
Hello, everyone:

Check out this quirky site: https://www.regiftable.com/regiftingrobinpopup.html

How does the computer always 'guess' correctly?

I'll check back to discuss...I saw this posted somewhere earlier today and immediately thought of the GMAT and how GMAT math skills can help you crack this code right away. If you're really intrigued, you can then ask yourself why the math rule that this engine depends upon holds true, too. Both thought processes should sharpen your GMAT math skills.
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Veritas Prep

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by kvcpk » Thu Sep 09, 2010 10:02 am
Brian@VeritasPrep wrote:Hello, everyone:

Check out this quirky site: https://www.regiftable.com/regiftingrobinpopup.html

How does the computer always 'guess' correctly?

I'll check back to discuss...I saw this posted somewhere earlier today and immediately thought of the GMAT and how GMAT math skills can help you crack this code right away. If you're really intrigued, you can then ask yourself why the math rule that this engine depends upon holds true, too. Both thought processes should sharpen your GMAT math skills.
Hey Brian,

Thats a good one. I saw a similar one during my college days. It was called the mystic ball.

My friend told me about it and I deduced the logic then.

2 digit number is 10X+Y
When we subtract X and Y from it, we get, 10X+Y-X-Y = 9(X)
Hence the resulting number is a multiple of 9.

Trick is that they keep all multiples of 9 as 'Box of choclates'

Good to see it again :)
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User avatar
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Posts: 1031
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Location: Malibu, CA
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by Brian@VeritasPrep » Thu Sep 09, 2010 4:00 pm
Wow - nice quick response, kvcpk! And you're right on - the "pattern" in that game is that every multiple of 9 has the same answer, and the math rule that they have you use ensures that you'll have a multiple of 9.

The algebraic way of finding it was slick...honestly, I didn't even think of that. I just played with the numbers to realize that, for two digit numbers:

By subtracting the units digit, you break it down to just the tens (e.g. 38 - 8 gives you 30)
By subtracting the tens digit from 0, you end up that many places away from the next ten (e.g. 30-3 - 27)
Each multiple of 9 is that many units away from the next ten (9 is 1 away, 18 is 2 away, etc.), mainly because 9 itself is one away

Just the process of playing with patterns like that will help you to recognize and use them on the GMAT. One of my biggest pleas to all of my students is to always ask "why" when they see a pattern or rule in math, as training yourself to see the logic underlying each rule is a thought process that will prove valuable throughout the study and test-taking process.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep

Looking for GMAT practice questions? Try out the Veritas Prep Question Bank. Learn More.
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