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Question in the attachment

by eitijan » Sat Mar 12, 2016 8:20 am
Source: GMATPrep

I was able to mark the correct answer but it took me very long time to test for each option. Is there any other shorter way of doing this question?

My approach:

h(100) + 1 = (2+4+...+100) + 1
=> h(100) + 1 = 2(1+2+...+50) + 1
=>h(100) + 1 = 2(50)(51)/2 + 1
=>h(100) + 1 = 2551

Tested for all the prime factors in the range for each option.
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by Brent@GMATPrepNow » Sat Mar 12, 2016 8:45 am
For every positive even integer n, the function h(n) is defined to be the product of all even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100) + 1, the p is

A: Between 2 & 10
B: Between 10 & 20
C: Between 20 & 30
D: Between 30 & 40
E: Greater than 40
Important Concept: If integer k is greater than 1, and k is a factor (divisor) of N, then k is not a divisor of N+1
For example, since 7 is a factor of 350, we know that 7 is not a factor of (350+1)
Similarly, since 8 is a factor of 312, we know that 8 is not a factor of 313

Now let's examine h(100)
h(100) = (2)(4)(6)(8)....(96)(98)(100)
= (2x1)(2x2)(2x3)(2x4)....(2x48)(2x49)(2x50)
Factor out all of the 2's to get: h(100) = [2^50][(1)(2)(3)(4)....(48)(49)(50)]

Since 2 is in the product of h(100), we know that 2 is a factor of h(100), which means that 2 is not a factor of h(100)+1 (based on the above rule)

Similarly, since 3 is in the product of h(100), we know that 3 is a factor of h(100), which means that 3 is not a factor of h(100)+1 (based on the above rule)

Similarly, since 5 is in the product of h(100), we know that 5 is a factor of h(100), which means that 5 is not a factor of h(100)+1 (based on the above rule)

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Similarly, since 47 is in the product of h(100), we know that 47 is a factor of h(100), which means that 47 is not a factor of h(100)+1 (based on the above rule)

So, we can see that none of the primes from 2 to 47 can be factors of h(100)+1, which means the smallest prime factor of h(100)+1 must be greater than 47.

Answer = E

Cheers,
Brent
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by Brent@GMATPrepNow » Sat Mar 12, 2016 8:47 am
eitijan wrote:Source: GMATPrep

I was able to mark the correct answer but it took me very long time to test for each option. Is there any other shorter way of doing this question?

My approach:

h(100) + 1 = (2+4+...+100) + 1
=> h(100) + 1 = 2(1+2+...+50) + 1
=>h(100) + 1 = 2(50)(51)/2 + 1
=>h(100) + 1 = 2551

Tested for all the prime factors in the range for each option.
You have misinterpreted the question.
h(n) is defined to be the product of all even integers from 2 to n
So, h(100) + 1 = (2)(4)(6)(8)....(96)(98)(100) + 1

Cheers,
Brent
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by eitijan » Sat Mar 12, 2016 8:56 am
Good catch!
My bad.
I completely overlooked this mistake.
Thanks.
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by [email protected] » Sat Mar 12, 2016 10:14 am
Hi eitijan,

This question shows up every so often in this Forum and it is definitely tougher than a typical GMAT question.

As a general rule, Quant questions are almost always based on a pattern of some kind (math formula, math rule, Number Property, etc.). If you can't immediately deduce a pattern, then you might have to "play around" a bit with the question to try to deduce what the pattern is. In the broad sense, it's critical thinking: here's a weird situation - what can I do to figure it out?

Based on the description of the function in the prompt, we can run some "TESTS" to try to figure things out....

The H(n) is the product of all the even integers from 2 to n, inclusive.

So....
H(4) = 2x4 = 8
The prime factors of 8 are (2)(2)(2)
If we do H(4) + 1 = 9, then the prime factors are (3)(3)
NOTICE how NONE of the prime factors of 8 are in 9? That's interesting....

H(6) = 2x4x6 = 48
The prime factors of 48 are (2)(2)(2)(2)(3)
If we do H(6) + 1 = 49, then the prime factors are (7)(7)
NOTICE how NONE of the prime factors of 48 are in 49? That's interesting....and probably a pattern, since it's happened TWICE NOW.

From here, I'd have to deduce that this pattern holds true. With H(100), I know that there are LOTS of primes that go in (the largest of which is 47, which can be "found" in 94). I have to assume that NONE of them will go into H(100) + 1. Thus, the smallest prime would have to be greater than 47. The question doesn't actually ask us for the exact answer though (the answer choices are "ranges").

The takeaway from all of this is that you shouldn't be afraid to "play" with a question a bit. In the end, you don't have to be a brilliant mathematician to answer this question, but you're also not allowed to just stare at it either.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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