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Elvin has sent wish lists to both Santa and Mrs. Claus. What is the probability that he will receive gifts from neither of them?
> This means we need to find P{Neither} or 1 - P{Santa} - P{Clause} + P{Both}.

(1) The probability that he will receive gifts from both Santa and Mrs. Claus is 1/4.
> P{Both} = 1/4. But this does not talk about either {Santa} or {Clause} which is required for finding P{Neither}.
Not sufficient.

(2) The probability that he will receive gifts from exactly one of them is three times the probability that he will receive gifts from both.
> P{Santa} + P{Clause} - 2P{Both} = 3 * P{Both}
> P{Santa} + P{Clause} = P{Both}
This would simplify the equation 1 - P{Santa} - P{Clause} + P{Both} to 1 - 2 * P{Both}. But this is still not sufficient.

Using (1) and (2), we can say P{Neither} = 1 - 2 * 1/4 => 1/2.
Hence (c).
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Bunuel
12 Days of Christmas 🎅 GMAT Competition with Lots of Questions & Fun

Elvin has sent wish lists to both Santa and Mrs. Claus. What is the probability that he will receive gifts from neither of them?

(1) The probability that he will receive gifts from both Santa and Mrs. Claus is 1/4.
(2) The probability that he will receive gifts from exactly one of them is three times the probability that he will receive gifts from both.

 


This question was provided by GMAT Club
for the 12 Days of Christmas Competition

Win $40,000 in prizes: Courses, Tests & more

 


------|--------S -------|------- NS --------|--------------
C
------|------------------|-------------------|--------------
NC
------|------------------|-------------------|--------------
********************************** 1
------|------------------|-------------------|-------------

(1) The probability that he will receive gifts from both Santa and Mrs. Claus is 1/4.

------|--------S -------|------- NS --------|--------------
C ******* 1/4
------|------------------|-------------------|--------------
NC
------|------------------|-------------------|--------------
********************************** 1
------|------------------|-------------------|-------------

The statement is not sufficient to find the other cells.

(2) The probability that he will receive gifts from both Santa and Mrs. Claus is 1/4.

------|--------S -------|------- NS --------|--------------
C ******** x
------|------------------|-------------------|--------------
NC
------|------------------|-------------------|--------------
********************************** 1
------|------------------|-------------------|-------------

x + 3x + N = 1

As we do not know the value of x, we can't find the value of N. The statement is not sufficient.

(1) + (2)

From (1) we know the value of x.

Hence, the statement combined can help us find the value of N.

Sufficient.

IMO C
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What We Need to Answer the Question

To find P(neither), we need to know the complete probability distribution:
  • P(both Santa AND Mrs. Claus)
  • P(exactly one of them)
  • P(neither of them)
P(both) + P(exactly one) + P(neither) = 1
Therefore: P(neither) = 1 - P(both) - P(exactly one)

Statement (1) Analysis: "P(both) = 1/4"
What This Tells Us:

  • P(both Santa AND Mrs. Claus) = 1/4
  • The remaining probability is 3/4
  • P(exactly one OR neither) = 3/4
  • But we cannot determine
    • P(exactly one) = ?
    • P(neither) = ?
      Statement (1) alone: NOT SUFFICIENT
Statement (2) Analysis: "P(exactly one) = 3 × P(both)"
What This Tells Us:

P(exactly one) = 3 × P(both)
P(neither) = 1 - P(both) - P(exactly one) = 1 - P(both) - 3×P(both) = 1 - 4×P(both)
What We Still Need:
The actual value of P(both)
Statement (2) alone: NOT SUFFICIENT

Statements (1) and (2) Together
What We Have:
P(both) = 1/4 [from Statement 1]
P(exactly one) = 3 × P(both) [from Statement 2]
P(exactly one) = 3 × (1/4) = 3/4
P(neither) = 1 - P(both) - P(exactly one) = 1 - 1/4 - 3/4 = 0

Both statements together: SUFFICIENT
ANSWER C







Bunuel
12 Days of Christmas 🎅 GMAT Competition with Lots of Questions & Fun

Elvin has sent wish lists to both Santa and Mrs. Claus. What is the probability that he will receive gifts from neither of them?

(1) The probability that he will receive gifts from both Santa and Mrs. Claus is 1/4.
(2) The probability that he will receive gifts from exactly one of them is three times the probability that he will receive gifts from both.

 


This question was provided by GMAT Club
for the 12 Days of Christmas Competition

Win $40,000 in prizes: Courses, Tests & more

 

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