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The numbers {1, 3, 6, 7, 7, 7} are used to form three

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by guerrero » Sat Sep 28, 2013 11:27 am
The numbers {1, 3, 6, 7, 7, 7} are used to form three 2-digit numbers. If the sum of these three numbers is a prime number p, what is the largest possible value of p?
A. 97
B. 151
C. 209
D. 211
E. 219

OA [spoiler]D

How should I tackle such questions ? Kindly help .[/spoiler]
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by [email protected] » Sat Sep 28, 2013 11:55 am
Hi guerrero,

This type of question requires a mix of "brute force", Number Properties and TESTING THE ANSWERS.

Here, we're told to form three 2-digit numbers (with the digits 1,3,6,7,7,7) that add up to a PRIME. We want the LARGEST POSSIBLE PRIME.

Let's take a look at the answers. We'll start with the largest:

219

We can quickly eliminate this answer because it's NOT PRIME (it's divisible by 3).

So, try the next largest number:

211

It IS prime. We know that to get to 211, we'll need some "bigger" 2-digit numbers. Playing around with the digits a bit, we can get:

67 + 71 + 73 = 211

This answer fits everything that we're told.

Final Answer: D

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