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BrentGMATPrepNow
Bunuel
If n is a prime number, does n = 17?

(1) n − 1 = m⁴, where m is an integer.

(2) n² < 300


Given: n is a prime number

Target question: Does n = 17?

Statement 1: n − 1 = m⁴, where m is an integer.
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several values of n and m that satisfy statement 1 (and the given information, which says n is prime). Here are two:
Case a: n = 2 and m = 1. In this case, the answer to the target question is NO, n does not equal 17
Case b: n = 17 and m = 2. In this case, the answer to the target question is YES, n equals 17
Since we can’t answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: n² < 300
Pro tip: Rather than try to find values of n that satisfy statement 2, we can save some time by checking to see whether we can REUSE any of the values we used for statement 1.
In this case, it turns out that we can reuse both pairs values, since the n-values in both pairs also satisfy statement 2 (i.e., 2² < 300 and 17² < 300)

So, we have:
Case a: n = 2. In this case, the answer to the target question is NO, n does not equal 17
Case b: n = 17. In this case, the answer to the target question is YES, n equals 17
Since we can’t answer the target question with certainty, statement 2 is NOT SUFFICIENT


Statements 1 and 2 combined
IMPORTANT: Notice that I was able to use the same counter-examples to show that each statement ALONE is not sufficient. So, the same counter-examples will satisfy the two statements COMBINED.
In other words,
Case a: n = 2 and m = 1. In this case, the answer to the target question is NO, n does not equal 17
Case b: n = 17 and m = 2. In this case, the answer to the target question is YES, n equals 17
Since we can’t answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer: E

Cheers,
Brent

I understood the explanation but while solving i multiplied 10 with 1.7 (root 3 value) by taking square of n to the 300 on the other side and got n < 17 and marked B. What is the flaw in the approach?

n < 300^1/2
n < 10 root 3
n < 17
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khushimahesh
BrentGMATPrepNow
Bunuel
If n is a prime number, does n = 17?

(1) n − 1 = m⁴, where m is an integer.

(2) n² < 300


Given: n is a prime number

Target question: Does n = 17?

Statement 1: n − 1 = m⁴, where m is an integer.
This statement doesn't FEEL sufficient, so I'll TEST some values.
There are several values of n and m that satisfy statement 1 (and the given information, which says n is prime). Here are two:
Case a: n = 2 and m = 1. In this case, the answer to the target question is NO, n does not equal 17
Case b: n = 17 and m = 2. In this case, the answer to the target question is YES, n equals 17
Since we can’t answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: n² < 300
Pro tip: Rather than try to find values of n that satisfy statement 2, we can save some time by checking to see whether we can REUSE any of the values we used for statement 1.
In this case, it turns out that we can reuse both pairs values, since the n-values in both pairs also satisfy statement 2 (i.e., 2² < 300 and 17² < 300)

So, we have:
Case a: n = 2. In this case, the answer to the target question is NO, n does not equal 17
Case b: n = 17. In this case, the answer to the target question is YES, n equals 17
Since we can’t answer the target question with certainty, statement 2 is NOT SUFFICIENT


Statements 1 and 2 combined
IMPORTANT: Notice that I was able to use the same counter-examples to show that each statement ALONE is not sufficient. So, the same counter-examples will satisfy the two statements COMBINED.
In other words,
Case a: n = 2 and m = 1. In this case, the answer to the target question is NO, n does not equal 17
Case b: n = 17 and m = 2. In this case, the answer to the target question is YES, n equals 17
Since we can’t answer the target question with certainty, the combined statements are NOT SUFFICIENT

Answer: E

Cheers,
Brent

I understood the explanation but while solving i multiplied 10 with 1.7 (root 3 value) by taking square of n to the 300 on the other side and got n < 17 and marked B. What is the flaw in the approach?

n < 300^1/2
n < 10 root 3
n < 17


I did the same thing. How is this wrong ?
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prachisaraf
khushimahesh
I understood the explanation but while solving i multiplied 10 with 1.7 (root 3 value) by taking square of n to the 300 on the other side and got n < 17 and marked B. What is the flaw in the approach?

n < 300^1/2
n < 10 root 3
n < 17


I did the same thing. How is this wrong ?

The point is \(\sqrt{3}\) is not exactly 1.7, it's an irrational number greater than 1.7: \(\sqrt{3} = 1.73205...\).
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