Stardust Chris wrote:
Is 3 a factor of the positive integer m?
(1) 12 is a factor of 25m
(2) 12 is a factor of 15m
Target question: Is 3 a factor of the positive integer m?This is a good candidate for
rephrasing the target question.
-----ASIDE---------------------
A lot of integer property questions can be solved using prime factorization.
For questions involving divisibility, divisors, factors and multiples, we can say:
If N is a factor by k, then k is "hiding" within the prime factorization of NConsider these examples:
3 is a factor of 24, because 24 = (2)(2)(2)
(3), and we can clearly see the
3 hiding in the prime factorization.
Likewise,
5 is a factor of 70 because 70 = (2)
(5)(7)
And
8 is a factor of 112 because 112 = (2)
(2)(2)(2)(7)
And
15 is a factor of 630 because 630 = (2)(3)
(3)(5)(7)
-----BACK TO THE QUESTION!---------------------
The above concept allows us to REPHRASE the target question as...
REPHRASED target question: Is there a 3 hiding in the prime factorization of m? Statement 1: 12 is a factor of 25m 12 = (2)(2)(3)
In other words, statement 1 is telling us that there are two 2's and one 3 hiding in the prime factorization of 25m
25m = (5)(5)(m)
Since there are no 3's hiding in (5)(5), it must be the case that
there's a 3 hiding in the prime factorization of mSince we can answer the
REPHRASED target question with certainty, statement 1 is SUFFICIENT
Statement 2: 12 is a factor of 15m 12 = (2)(2)(3)
In other words, statement 2 is telling us that there are two 2's and one 3 hiding in the prime factorization of 15m
15m = (
3)(5)(m)
As we can see, we already have a 3 hiding in the prime factorization of 15m, so we can't say for sure whether there's a 3 hiding in the prime factorization of m.
To more certain, consider these two counter-examples:
Case a: m = 4. This means 15m = (15)(4) = 60, and 12 IS a factor of 60. In this case,
3 is NOT a factor of mCase b: m = 12. This means 15m = (15)(12) = 180, and 12 IS a factor of 180. In this case,
3 IS a factor of mSince we cannot answer the
target question with certainty, statement 2 is NOT SUFFICIENT
Answer: A
Cheers,
Brent
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