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The task at hand is to identify whether x and y are of different signs i.e. positive or negative.

Analyzing statement (1) -x + y > 5
It is very simple to plug in values of x and y to yield different answers.
(x=1, y=7) satisfy the statement and their product yields a positive value, while (x=-1,y=7) satisfy the statement and their product yields a negative value.
Thus from statement (1), we cannot state for sure whether xy < 0.

Analyzing statement (2) 3y - x < -9
Multiply both sides by -1, it can be rephrased as x - 3y > 9 (remember to change the inequality sign)
Now plugging values
(x=13, y=1) satisfy the statement and their product yields a positive value, while (x=13,y=-1) satisfy the statement and their product yields a negative value.
Thus from statement (2), we cannot state for sure whether xy < 0.

Combining the two statements
-x + y > 5 and x - 3y > 9
Adding the two, we get
-2y > 14
Divide by 2, we get
-y > 7 or y < -7 i.e. y is negative
Using the above in statement (1) -x + y > 5, we get
-x > 12 or x < -12, i.e. both x and y are negative.
Hence we can state assertively that the product of x and y is positive, i.e. xy is not less than 0.

Correct answer C.
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Is xy < 0?

(1) -x + y > 5
(2) 3y - x < -9
Explanation:-
1)-x + y > 5 -> y>x+5 .suppose x=1,y=7 xy >0,but x=-1,y=3 xy<0 hence insufficient.
2)3y - x < -9 -> x>3y+9.suppose y=0,x=10 xy =0,but y=-1,x=7 xy<0 hence insufficient

Combining ,a and b
y>x+5 and x>3y+9 -> y>x+5>(3y+5)+9 -> y>3y+14 ->y<-7.Because y is greater than x ,x and y are always negative Hence XY >0.Sufficient.

Hence Both statements together are sufficient. Answer is "C".
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Note that with a degree 1 and 2 variables inequalities a single equation cannot solve the problem, which mean that 1 and 2 alone is not sufficient. Just need to figure out if 1 and 2 together is sufficient or not.
Most common way is draw 2 linear equations and pick the answer. But with this type of problem I can do that :
y - x > 5 > 0 and 3y - x < -9 < 0 => (y-x)*(3y-x) < 0 => 3y^2 - xy - 3xy + x^2 < 0 => 3y^2 + x^2 -4xy < 0
3y^2 + x^2 >= 0 with every x and y => 4xy > 3y^2 + x^2 >= 0, or xy > 0. The answer is no => 1 and 2 together is sufficient.
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Answer: C

1) -x + y > 5 is equivalent to y - x > 5
We can test for different values for x and y that will meet the condition.
I. x > 0 and y > 0. Ex. x = 2; y = 8 meet the criteria 6 > 5. This results in x * y > 0
II. x < 0 and y > 0. Ex. x = - 8 and y = 2. Again 6 > 5. This results in -x * y < 0
Two different results, therefore insufficient.

2) 3y - x < -9. This is equivalent to x - 3y > 9
Again, we can test for different values for x and y that will meet the condition.
I. x > 0; y < 0. Ex. x = 10; y = -1 => 13 > 9. Therefore x*y is negative
II. x > 0; y > 0. Ex. x = 20; y = 1 => 17 > 9. Therefore x*y is positive
Two different results, therefore insufficient.

Combining both inequalities:
y - x > 5
x - 3y > 9

Summing these two, we get:

y - x + x - 3y > 5 + 9
-2y > 14
y < -7
Therefore, y is negative.

(#1) Let's pick a valid value for y = - 8. Substituting for y in the first equation, we get:
- 8 - x > 5
- x > 13
x < -13

(#2) Let's substitute y = -8:

x -3 * - 8 > 9
x + 24 > 9
x > - 15

From #1 and #2 we have -13 > x > -15. Therefore, we know for a certain that x is negative.

Finally, we know that x and y are negative, therefore x * y is positive.
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Basically the question is "Do x and y have different signs?"

S1: \(-x + y > 5\)
Plugging values (Same signs):
x=1; y=10
\(-1 + 10 = 9 > 5\)
Answer: No

Plugging values (Different signs):
x=-10; y=1
\(-(-10) + 1 = 11 > 5\)
Answer: Yes

Hence, Insufficient

S2: \(3y - x < -9\)
Plugging values (Same signs):
x=-1; y=-5
\(3(-5) - (-1) = -14 < -9\)
Answer: No

Plugging values (Different signs):
x=5; y=-5
\(3(-5) - (5) = -20 < -9\)
Answer: Yes

Hence, Insufficient

S1+S2:
(1) -x + y > 5
(2) 3y - x < -9

S1=> \(-x + y > 5\) => \(x - y < -5\) (multiplying with negative value flips the inequality)
S2=> \(3y - x < -9\)

S1 + S2 =
\(x - y < -5\)
\(-x + 3y < -9\)
_________
\(2y < -14\)
\(y < -7\) ----i

Adding S1 and i
\(x - y < -5\)
\(y < -7\)
________
\(x < -12\) ----ii

hence from i and ii,\(x<-12\) and \(y<-7\).
Both x and y have same signs
Hence Sufficient

Answer C
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Bunuel

Jamboree and GMAT Club Contest Starts



QUESTION #2:

Is xy < 0?

(1) -x + y > 5
(2) 3y - x < -9

Check conditions below:


For the following two weekends we'll be publishing 4 FRESH math questions and 4 FRESH verbal questions per weekend.

To participate, you will have to reply with your best answer/solution to the new questions that will be posted on Saturday and Sunday at 9 AM Pacific.
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JAMBOBREE OFFICIAL SOLUTION:

In this question we need to calculate whether the product of X and Y is negative. In other words we need to know the sign of x and Y.
In the statement (1) we can say that Y> x + 5. But we do not know the sign of x so we cannot conclude any thing about he sign of Y.
Similarly In the statement (2) we can say that Y< (x -9)/3. But we not know the sign of x sowe cannot conclude any thing about he sign of Y

Multiplying 2nd inequality by -1 gives: -3Y + X > 9. Now both the inequalities have same sign (‘>’), hence the 2 inequalities can be added. We get: Y < -7 and x < -12. Hence both X and Y are negative, hence the product of x and y will surely be greater than 0 and we will have a definite NO as the definite answer for the question asked.

Answer: C.
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xy<0
So we have to prove these two are of opposite signs, or disprove that, whichever possible.

(1) -x+y>5
Rearranging, we get
y>x+5
Since y > x+(positive value), we get y>x. But, this does not tell us about their signs. Insufficient

(2) 3y-x<-9
Rearranging, we get
x>9+3y, which is similar to above statement. Insufficient

So we add both statements.

-x + y > 5
-3y+x >9(Multiplying by -1 throughout so we can add same ineqalities while cancelling x at the same time)

Thus,
-2y>14
or, y<-7(which is ALWAYS negative)

Also, we know y>x+5. So x is even more negative than y for all possible values. So xy will always be positive. So xy<0 is not true and can be found using both statements.

So C
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tarunktuteja
Is xy < 0?

(1) -x + y > 5
(2) 3y - x < -9


This can be easily done using graphs. Look at the attached graph:

(1) -x + y > 5
It can be seen that the graph of this equation that this equation has solutions in I, II and III quadrants. For I and III quadrants, xy>0 while for II quadrant, xy<0. Hence insufficient.

(2) 3y - x < -9
It can be seen that the graph of this equation that this equation has solutions in I, IV and III quadrants. For I and III quadrants, xy>0 while for IV quadrant, xy<0. Hence insufficient.

(1)+(2)
It can be seen that the graphs for 1 and 2 overlaps in III quadrant only and for III quadrants, xy>0, hence it can concluded that xy is not less than 0. Hence sufficient.


Great Work..!
Just learnt a new perspective ..
Thanks..!!
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Vyshak
Is xy negative?

1) y - x > 5
6 - 0 > 5 --> xy = 0
6 - (-1) > 5 --> xy = -6
So 1 alone is not sufficient

2) 3y - x < -9
3(-4) - 0 < -9 --> xy = 0
3(-4) - 1 < -9 --> xy = -4
So 2 alone is not sufficient

Combining 1 and 2,

Multiply St.1 by 3 on both sides,
3y - 3x > 15
3y - x - 2x > 15
From St. 2 we have, 3y - x < -9.. Assume 3y - x = -9
-9 - 2x > 15
-2x > 24
x < -12

If x < -12,
3y - x < -9.. Assume the value of x to be -12
3y - (-12) < -9
3y < -21 --> y < -7.

Combining St1 and St2 we get that both x and y are negative. Product of 2 negative numbers is positive. So xy is non-negative.

Answer: C

In your solution, you are assuming that 3y-x=-9 whereas actual given statement is that 3y-x<-9. So even if we assume initially, while arriving at the answer at a later stage, arent we supposed to bring the solution back to the original statement minus assumption?
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