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Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8027
GMAT 1: 760 Q51 V42 GPA: 3.82
If x and y are integers, is 3x^2+5x+y an even number?  [#permalink]

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Difficulty:   35% (medium)

Question Stats: 67% (01:14) correct 33% (01:07) wrong based on 76 sessions

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[GMAT math practice question]

If x and y are integers, is $$3x^2+5x+y$$ an even number?

1) $$x=5$$
2) $$y=4$$

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Math Expert V
Joined: 02 Aug 2009
Posts: 8006
Re: If x and y are integers, is 3x^2+5x+y an even number?  [#permalink]

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MathRevolution wrote:
[GMAT math practice question]

If x and y are integers, is $$3x^2+5x+y$$ an even number?

1) $$x=5$$
2) $$y=4$$

$$3x^2+5x$$ will always be EVEN irrespective of value of x..
If x is odd, both 3x^2 and 5x making odd+odd=even..
If X is even, both are even and E+E=E..
So the equation depends on y..
1) x=5..
Insufficient
2) y=4
So 3x^2+5x+y=even + even=Even
Suff

B
_________________
Math Revolution GMAT Instructor V
Joined: 16 Aug 2015
Posts: 8027
GMAT 1: 760 Q51 V42 GPA: 3.82
Re: If x and y are integers, is 3x^2+5x+y an even number?  [#permalink]

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=>

Forget conventional ways of solving math questions. For DS problems, the VA (Variable Approach) method is the quickest and easiest way to find the answer without actually solving the problem. Remember that equal numbers of variables and independent equations ensure a solution.

Since we have $$2$$ variables ($$x$$ and $$y$$) and $$0$$ equations, C is most likely to be the answer. So, we should consider conditions 1) and 2) together first.

Conditions 1) and 2):
Since $$x = 5$$ and $$y = 4$$, we have $$3x^2+5x+y = 3*5^2 + 5*5 + 4 = 75 + 25 + 4 = 104$$, which is even.
Thus, both conditions together are sufficient.

Since this is an integer question (one of the key question areas), we should also consider choices A and B by CMT 4(A).

Condition 1):
If $$x = 5$$ and $$y = 1$$, then
$$3x^2+5x+y =3*5^2 + 5*5 + 1 = 75 + 25 + 1 = 101$$, which is odd.
If x = 5 and y = 4, then
$$3x^2+5x+y =3*5^2 + 5*5 + 4 = 75 + 25 + 4 = 104$$, which is even.
Since we do not have a unique answer, condition 1) is not sufficient.

Condition 2)
There are two cases to consider.
Case 1: x is even.
Since $$3x^2$$ and 5x are even and y is even, $$3x^2+5x+y$$ is even.
Case 2: $$x$$ is odd.
Since $$3x^2+5x$$ is even and $$y$$ is even, $$3x^2+5x+y$$ is even.
Since we have a unique answer, condition 2) is sufficient.

Therefore, the answer is B.

Normally, in problems which require 2 equations, such as those in which the original conditions include 2 variables, or 3 variables and 1 equation, or 4 variables and 2 equations, each of conditions 1) and 2) provide an additional equation. In these problems, the two key possibilities are that C is the answer (with probability 70%), and E is the answer (with probability 25%). Thus, there is only a 5% chance that A, B or D is the answer. This occurs in common mistake types 3 and 4. Since C (both conditions together are sufficient) is the most likely answer, we save time by first checking whether conditions 1) and 2) are sufficient, when taken together. Obviously, there may be cases in which the answer is A, B, D or E, but if conditions 1) and 2) are NOT sufficient when taken together, the answer must be E.

_________________ Re: If x and y are integers, is 3x^2+5x+y an even number?   [#permalink] 07 Jan 2018, 18:13
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