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Solution


Given:
    • xy < 0
      o Implies, either x < 0 or y < 0

To find:
    • The value of |x + y|

Analysing Statement 1
|x| = 4
    • Implies, x = 4 or -4
    • However, we do not have any information about y

Therefore, statement 1 is not sufficient

Analysing Statement 2
|y| = 1
    • Implies, y = 1 or -1
    • However, we do not have any information about x

Therefore, statement 2 is not sufficient

Combining Both Statements
Combining both statements, we get,
    • x = ±4 and y = ±1
    • Thus,
      o If x = 4, then y = -1
         Implies, |x + y| = |4 – 1| = 3

      o If x = -4, then y = 1
         Implies, |x + y| = |-4 + 1| = |-3| = 3

Therefore, both statements together are sufficient

Hence, the correct answer is Option C.

Answer: C

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Bunuel
If xy < 0, what is the value of |x + y| ?

(1) |x| = 4
(2) |y| = 1


for xy < 0 to be valid
either x or y has to be -ve

from 1 : lxl can be +4 or -4 , y not given not sufficient

from 2 : lyl can be +/-1 , x not given not sufficient

from 1 or 2 substitutting either values of x or y as + or -ve we would get

l-4+1l = l4-1l

which would be 3

IMO C sufficient
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Bunuel
If xy < 0, what is the value of |x + y| ?

(1) |x| = 4
(2) |y| = 1

Par of GMAT CLUB'S New Year's Quantitative Challenge Set

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Bunuel
If xy < 0, what is the value of |x + y| ?

(1) |x| = 4
(2) |y| = 1

(1) |x| = 4. Clearly insufficient because we do not know anything about y.
(2) |y| = 1. Clearly insufficient because we do not know anything about x.

(1) + (2):
We have |x| > |y|, and xy have opposite signs (because xy < 0) --> |x + y| = |x| - |y| = 4 - 1 = 3. SUFFICIENT.

Answer: (C)
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(1)
|x| = 4 or - 4

However, we dont know value of y. Insufficient.


(2)
|y| = 1 or -1

However, we dont know value of x. Insufficient.


(1) and (2),

and also xy<0 - meaning (+)(-) or (-)(+) for x and y respectively.

|-4+1| = 3
|4-1| = 3

Answer is C.
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