If k, n, and m are positive integers, is kn even?

Question

(1) k = m^3
(2) n = (m - 1)^3

lacktutor · Answer

If k, n, and m are positive integers, is kn even?

Statement1: k =  m^{3} 
if m is odd, then  m^{3}  will be odd, --> k is odd
if m is even, then  m^{3}  will be even--> k is even. 
Also, No info about what n is.
Insufficient

Statement2: n =  (m - 1)^{3} 
If m is odd, then  (m-1)^{3}  will be even --> n is even
if m is even, then  (m-1)^{3}  will be odd--> n is odd.
Also, NO info about what k is.
Insufficient

Taken together 1 and 2, 
In order kn to be even, one of k or n should be even.
If m is even, then (m-1) will be odd--> k= m^{3} -even, n= (m-1)^{3} -odd
--> Even*Odd=Even

If m is odd, then (m-1) will be even--> k= m^{3} -odd, n= (m-1)^{3} -even 
--> Odd*Even=Even

in both cases, kn is EVEN
sufficient
The answer is C.

Archit3110 · Answer

#1
n not know insufficient
#2
k not know insufficient
from 1 &2
k*n ; m may be odd or even but k*n will always be even sufficient
IMO C

greenisthecolor · Answer

(1) k = m^3

k can be odd or even depending on the value of m. Also, we have no information about n. So kn can be even or odd. INSUFFICIENT. Eliminate A and D.

(2) n = (m - 1)^3

Similar to (1), n can be odd or even depending on the value of m. Also, we have no information about n. So kn can be even or odd. INSUFFICIENT. Eliminate B.

Combining (1) and (3):

kn =  (m * (m-1))^3 , which is cube of product of two consecutive integers. Product of two consecutive integers is even => cube of even is also even => kn is even. Answer is C.

Note: kn cannot be 0 since both k and n are positive integers.

Bunuel · Answer

Official Explanation

We want to know whether kn is even, and both k and n are integers, so it will be sufficient to know that k or n is even, since the product of the two will therefore include a factor of 2. On to the data statements, separately first.

Statement (1) tells us about k and m. If m is even, k is even. If m is odd, k is odd. But we don't know whether m is even or odd, so we can't draw any conclusions. The even/oddness of both k and n are still unconstrained. Statement (1) is insufficient.

Statement (2) is similar to Statement (1) in that it relates one of our variables of interest (this time n) to m. If m is odd, then n is even. If m is even, then n is odd. But we don't know whether m is odd or even, and we have no constraint or conclusion about k, so this statement is also insufficient.

Combining the statements, we can combine conclusions. Statement (1) told us that k had the same even/odd-ness as m. Statement (2) told us that n had the opposite even/odd-ness as m. Therefore, we know that, regardless of whether m is even or odd, k and n will have opposite even/odd-ness. That means that one of the two will be even (while the other is odd), and their product will always be even. We can answer the question definitively, so the statements give sufficient information when taken together.

The correct answer is (C).

Bunuel · Answer

Video Explanation

xb_LAoLks6g

PeeKaBoo11 · Answer

Statement 1: (insufficient)
Nothing is known about "n"

Statement 2: (insufficient)
Nothing is known about "k"

On combining statement 1 and Statement 2

No matter what value m takes either of (m-1) or m will be even.
Also, m cannot be equal to 0. As this would make n = (-1) ....{statement 2}
So by combining we can conclusively say that k*n will be even as Even * Odd = Odd* Even = Even.

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

If k, n, and m are positive integers, is kn even?

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia