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.