If n and y are positive integers and 450y = n³, which of the following must be an integer?
I. y/(3 x 2² x 5)
II. y/(3² x 2 x 5)
III. y/(3 x 2 x 5²)
a. None
b. I only
c. II only
d. III only
e. I, II, and III
While solving this question, I arrived at the correct answer as b then thought the answer could be e. OA is b
Here is the approach, which led me to select the answer as e
450y must be the cube root and factoring 450 we get;
450 = 2 x 3² x 5²
so value of y can be 3 x 2² x 5
Thus I is an integer in this case.
Also y can have other values such as y = (3 x 2² x 5) x (3³ x 2³ x 5³) in which case II & III could also be true.
Please confirm the issue with this approach.
GMAT Prep: integer
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It almost always helps to find the prime factorization in these question types where we ask whether a certain rational expression is an integer.psm12se wrote:If n and y are positive integers and 450y = n³, which of the following must be an integer?
I. y/(3 x 2² x 5)
II. y/(3² x 2 x 5)
III. y/(3 x 2 x 5²)
a. None
b. I only
c. II only
d. III only
e. I, II, and III
450y = n^3
2*3*3*5*5*y = n^3
For 2*3*3*5*5*y to be a cube, we need the number of 2's, 3's and 5's in the prime factorization to each be divisible by 3.
So, for example, 2*2*2*2*2*2*3*3*3*5*5*5 = (2*2*3*5)^3
For 2*3*3*5*5*y to be a cube, it must be the case that the prime factorization of y includes at least two additional 2's, one additional 3 and one additional 5.
So, y = 2*2*3*5*(other possible numbers)
Now check the option.
I. Must y/(3 * 2^2 * 5) be an integer?
Plug in y to get: 2*2*3*5*(other possible numbers)/(3 * 2^2 * 5)
= some integer
Since this must be an integer, we can eliminate A, C and D, which leaves us with B or E.
II. Must y/(3^2 * 2 * 5) be an integer?
Plug in y to get: 2*2*3*5*(other possible numbers)/(3^2 * 2 * 5)
= 2*(other possible numbers)/3
Not necessarily an integer
Since this need not be an integer, we can eliminate E, which leaves us with B.
NOTE: At this point we have the correct answer. But let's check III for "fun"
III. Must y/(3 * 2 * 5^2) be an integer?
Plug in y to get: 2*2*3*5*(other possible numbers)/(3 * 2 * 5^2)
= 2*(other possible numbers)/5
Not necessarily an integer
Answer: B
Cheers,
Brent
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Good logic! Only one misstep: the question stem says MUST, not COULD - so each equation must work for ANY valid value of y.psm12se wrote:If n and y are positive integers and 450y = n³, which of the following must be an integer?
Also y can have other values such as y = (3 x 2² x 5) x (3³ x 2³ x 5³) in which case II & III could also be true.
Please confirm the issue with this approach.
As Brent demonstrated above, y must give you all the "missing" factors of a perfect cube, so the easiest way to find answers that MUST work is to find the minimum positive value of y, then test it. If an equation works for the minimum, it should work for every other value as well, as every other value (here) will just be a multiple of the minimum.
For instance, y could be 3 * 5 * 2 * 2 * 7 * 7 * 7 * 11 * 11 * 11, or whatever ... but it'll always be a multiple of 3 * 5 * 2 * 2, as those are the factors required to have three 3's, three 5's, and three 2's in the prime factorization of n (i.e. to give n three of each of its unique prime factors, making it a perfect cube).