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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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Bunuel wrote:
The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260


A prime number is a number that has only two factors: 1 and itself. Therefore, a prime number is divisible by two numbers only.

Let's list the numbers from 61 to 69.

61, 62, 63, 64, 65, 66, 67, 68, 69

Immediately we can eliminate the EVEN NUMBERS because they are divisible by 2 and thus are not prime.

We are now left with: 61, 63, 65, 67, 69

We can next eliminate 65 because 65 is a multiple of 5.

We are now left with 61, 63, 67, 69.

To eliminate any remaining values, we would look at those that are multiples of 3. If you don’t know an easy way to do this, just start with a number that is an obvious multiple of 3, such as 60, and then keep adding 3.

We see that 60, 63, 66, 69 are all multiples of 3 and therefore are not prime.

Thus, we can eliminate 63 and 69 from the list because they are not prime.

Finally, we are left with 61 and 67, and we must determine whether they are divisible by 7. They are not, and therefore they must be both prime. Thus, the sum of 61 and 67 is 128.

Answer B.

Here is a useful rule: If a two-digit number is a prime, it can’t be divisible by any of the single-digit primes. That is, it can’t be divisible by 2, 3, 5 and 7. In other words, if you have a two-digit number that is not divisible by 2, 3, 5 and 7, it must be a prime. If you have trouble seeing that 61 and 67 are prime, I would suggest that you review your multiplication tables. Doing so will allow you to quickly see that 61 and 67 are not multiples of a given single-digit number, such as 7.
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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SOLUTION

The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260

The only prime numbers between 60 and 70 are 61 and 67 --> 61+67=128.

Answer: B.
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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Just 2 prime numbers 61 & 67 in the given range

61+67 = 128

Answer = B

OA has listed 67; easy to find other :)
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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60<p<70
p=prime number/s
Prime numbers between 60 and 70 are 61 and 67.

62,64,66,68 (divisible by 2)
63,66,69(divisible by 3)
65(divisible by 5).

Therefore 61+67=128
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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Bunuel wrote:
The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260

Practice Questions
Question: 5
Page: 152
Difficulty: 550


Sum of any two numbers between 60 and 70 MUST BE

Greater than 60+60 i.e. 120
Less than 70+70 i.e. 140


Only Option B fits in the range :cool:
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
Bunuel wrote:
The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260

Practice Questions
Question: 5
Page: 152
Difficulty: 550



2 is the only even prime number. so 61,63,67,69 -- 63 & 69 both are divisible by 3 hence 61 & 67 =128..
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
[quote="Bunuel"]The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260



you may notice 61 and 67 are prime numbers here and the sum of them is 128
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
Any prime number can be expresses as 6n+1 (6n-1)

Let's start:

6*10+1= 61
6*11+1=67
6*12+1 - too much, so you can stop here.

61+67=128. Answer (B)
Re: The sum of prime numbers that are greater than 60 but less [#permalink]
Quote:
The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260

Here, the range of the number is 61 (lowest number of the range) to 79 (highest number of the range). If there is just one prime number then the answer would be A. But, in this question prompt, there are 2 prime numbers (61,67). The unit digit of the summation of 2 prime numbers is EVEN (considering the prime number 2 is not here in this range). So, choice A, C, D are out. The rest choices are B) 128; and E) 260. The summation of 2 prime numbers can't exceed 138 (\(79×2=138\)). So, E is out. The correct choice is B.


Here are the first few prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, etc.
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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Bunuel wrote:
The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260

Practice Questions
Question: 5
Page: 152
Difficulty: 550


The primes between 60 and 70 are: 61, 67

Sum = 61 + 67 = 128

Answer: B
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
Bunuel wrote:
The sum of prime numbers that are greater than 60 but less than 70 is

(A) 67
(B) 128
(C) 191
(D) 197
(E) 260

Practice Questions
Question: 5
Page: 152
Difficulty: 550


prime numbers >5 can be determined by 6n+/-1
so from 60 to 70
6*10+1 ; 61
and 6*11+1 ; 67
two numbers ; sum 61+67 ; 128
OPTION B
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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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We need to find the sum of prime numbers that are greater than 60 but less than 70

Prime numbers between 60 and 70 are 61, 67

=> Sum = 61 + 67 = 128

So, Answer will be B.
Hope it helps!

Watch the following video to learn How to find Prime Numbers from 1 to 100

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Re: The sum of prime numbers that are greater than 60 but less [#permalink]
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