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# If the sum of a set of ten different positive prime numbers

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Intern
Joined: 24 Feb 2013
Posts: 15
Location: Argentina
WE: Project Management (Energy and Utilities)
If the sum of a set of ten different positive prime numbers  [#permalink]

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Updated on: 23 May 2013, 08:15
2
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Difficulty:

5% (low)

Question Stats:

95% (00:56) correct 5% (02:16) wrong based on 93 sessions

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If the sum of a set of ten different positive prime numbers is an even number, which of the following prime numbers CANNOT be in the set?

A. 2
B. 3
C. 5
D. 7
E. 11

Originally posted by grotten on 23 May 2013, 07:33.
Last edited by Bunuel on 23 May 2013, 08:15, edited 1 time in total.
Renamed the topic and edited the question.
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23 May 2013, 07:35
2
Every prime number except 2 is odd.

If we sum $$10$$ odd numbers we get an even number.
If we sum $$9$$ odd numbers and $$1$$ even number we get an odd number.

Since the sum is even, all primes must be obb => no 2.
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Intern
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23 May 2013, 07:38
The set contains 10 numbers. Divide the 10 numbers in sets of two numbers.
Knowing that Odd+Odd = Even
Then the 5 - 2 number set would be:

(Odd + Odd) + (Odd + Odd) + (Odd + Odd) + (Odd + Odd) + (Odd + Odd) = Even + Even + Even + Even + Even = Even !!
This concludes that every number in the set must be Odd, because Odd + Even = Odd
The only even number is 2.
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Re: If the sum of a set of ten different positive prime numbers  [#permalink]

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14 Dec 2015, 00:31
grotten wrote:
If the sum of a set of ten different positive prime numbers is an even number, which of the following prime numbers CANNOT be in the set?

A. 2
B. 3
C. 5
D. 7
E. 11

All prime numbers apart from 2 are odd.
Even + Even = Even
Odd + Even = Odd
Odd + Odd = Even

We are given ten different prime numbers, whose sum is even
If we include 2, we will have 9 odd prime numbers and one even.
This sum would be odd

If we exclude 2, we will have 10 odd numbers.
This sum would be even

Hence 2 is not included.
Option A
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Re: If the sum of a set of ten different positive prime numbers  [#permalink]

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09 Jan 2018, 09:05
Top Contributor
grotten wrote:
If the sum of a set of ten different positive prime numbers is an even number, which of the following prime numbers CANNOT be in the set?

A. 2
B. 3
C. 5
D. 7
E. 11

Some important rules:
1. ODD +/- ODD = EVEN
2. ODD +/- EVEN = ODD
3. EVEN +/- EVEN = EVEN

4. (ODD)(ODD) = ODD
5. (ODD)(EVEN) = EVEN
6. (EVEN)(EVEN) = EVEN

The key concept here is that 2 is the only EVEN prime number. All other primes are ODD.
Since the set contains 10 different prime numbers , there are only two possible cases:
case 1) 2 is in the set of primes
case 2) 2 is NOT in the set of primes

case 1: If 2 IS in the set, then the set contains 9 ODD primes and 1 EVEN prime
In this case, the sum of the 10 primes will be ODD.
However, the question tells us that the sum is EVEN.
So, it CANNOT be the case that 2 is in the set of numbers.

Cheers,
Brent
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Re: If the sum of a set of ten different positive prime numbers &nbs [#permalink] 09 Jan 2018, 09:05
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