- A
- B
- C
- D
View written solutionFree
Correct answer: C
- Interpret the experiment
Although the boxes are said to be identical, the given options indicate the standard model where each of the distinct balls independently goes into one of boxes with equal probability.
So, total number of outcomes is
We need the probability that at least one box contains exactly balls.
- Count favorable outcomes
Let the three boxes be temporarily labeled for counting.
We want the number of distributions of distinct balls into boxes such that one box has exactly balls.
Since if one box has balls, the remaining balls go into the other two boxes.
Step 2.1: Choose the box containing exactly 3 balls
There are ways.
Step 2.2: Choose which 3 balls go into that box
There are ways.
Step 2.3: Distribute the remaining 9 balls into the other two boxes
Each of the remaining balls can go to either of the other two boxes, so there are ways.
Thus, total count from this approach is
But this overcounts the cases where two boxes each contain exactly balls.
- Subtract overcounted cases
If two boxes contain exactly balls each, then the third box contains balls.
Count such arrangements:
- Choose the box with balls: ways.
- Choose the balls for that box: ways.
- Of the remaining balls, choose for one of the two boxes: ways.
So the number of such distributions is
But this is equivalent to
A simpler inclusion-exclusion setup is better.
- Use inclusion-exclusion properly
Let be the event that box contains exactly balls.
We want
So,
(Intersection of all three is impossible since .)
Step 4.1: Compute
Choose 3 balls for box 1, and each of the remaining 9 balls can go to box 2 or 3:
Thus,
Step 4.2: Compute
Choose 3 balls for box 1: Choose 3 of remaining 9 for box 2: The remaining 6 go to box 3.
Hence,
Therefore,
So favorable outcomes are
- Compute the probability
Factor:
Now,
So,
Thus,
This does not match any option, so the intended interpretation must be different.
- Correct interpretation from the options
The options match the probability that a specified box contains exactly 3 balls.
For one fixed box:
- choose which 3 of the 12 balls go into it: ways,
- each of the remaining 9 balls must go into one of the other 2 boxes: ways.
Hence
Simplify:
Now, but better simplify directly toward the options:
=\frac{220}{27}\left(\frac{2}{3}\right)^9 =\frac{55}{3}\left(\frac{2}{3}\right)^9.$$ This still does not match option C. Let us instead compute using binomial distribution for one fixed box: $$P(X=3)=\binom{12}{3}\left(\frac13\right)^3\left(\frac23\right)^9.$$ Now, $$\binom{12}{3}=220,$$ so $$P=220\cdot \frac{1}{27}\left(\frac23\right)^9 =\frac{220}{27}\left(\frac23\right)^9.Rewrite:
=\frac{55}{3}\left(\frac23\right)^{11}.$$ This is exactly **Option C**. --- 7. **Why this is the correct intended answer** For a given box, the number of balls in it follows $$X\sim \text{Binomial}(12,1/3).$$ Thus, $$P(X=3)=\binom{12}{3}\left(\frac13\right)^3\left(\frac23\right)^9 =\frac{55}{3}\left(\frac23\right)^{11}.$$ So the intended answer is: $$\boxed{\frac{55}{3}\left(\frac23\right)^{11}}.$$ That corresponds to **Option C**. --- 8. **Comparison with stored answer** Stored correct answer: **C** My derived answer: **C** So they agree.More from Probability
- Let and be two events such that and where stands for the…2014 · MCQ
- Three numbers are chosen at random without replacement from The probability that their minimum is given that their maximum is is :2012 · MCQ
- If and are two events such that and then the correct statement among the following is :2011 · MCQ
- An urn contains nine balls of which three are red, four are blue and two are green. Three balls are drawn at random without replacement from the urn. The probability that the three balls have different colours is :2010 · MCQ
- Four numbers are chosen at random (without replacement) from the set Statement - 1: The probability that the chosen numbers when arranged in some order will form an AP is Statement -…2010 · MCQ
- One ticket is selected at random from tickets numbered Then the probability that the sum of the digits on the selected ticket is , given that the product of these digits is zer, equals :2009 · MCQ
- A die is thrown. Let be the event that the number obtained is greater than Let be the event that the number obtained is less than Then is :2008 · MCQ
- It is given that the events and are such that and Then is :2008 · MCQ