2020 AMC 12B Problems/Problem 16
Problem
An urn contains one red ball and one blue ball. A box of extra red and blue balls lie nearby. George performs the following operation four times: he draws a ball from the urn at random and then takes a ball of the same color from the box and returns those two matching balls to the urn.
After the four iterations the urn contains six balls. What is the probability that the urn contains three balls of each color?
Solution
Let the probability that the urn ends up with more red balls be denoted 
. Since this is equal to the probability there are more blue balls, the probability there are equal amounts is 
. 
the probability no more blues are chosen plus the probability only 1 more blue is chosen. The first case, 
.
The second case, 
. Thus, the answer is 
.
~JHawk0224
Solution 2
By conditional probability after 4 rounds we have 5 cases: RRRBBB, RRRRBB, RRBBBB, RRRRRB and BRRRRR. Thus the probability is 
. Put 
.
~FANYUCHEN20020715
Solution 3
Here X stands for R or B, and Y for the remaining color.
After 5 rounds one can either have a 4+1 configuration (XXXXY), or 3+2 configuration (XXXYY). The probability of getting to XXXYYY from XXXYY is 
. Observe that the probability of arriving to XXXXY configuration is 
( 
to get from XXY to XXXY, 
to get from XXXY to XXXXY). Thus the probability of arriving to XXXYY is also 
, and the answer is ![\[\frac{1}{2} \cdot \frac{2}{5} = \boxed{\textbf{(B)}\ \frac{1}{5}}.\]](http://latex.artofproblemsolving.com/c/9/9/c99ff5ad41662fef1424b2b5bca161b2a5b38bea.png)
See Also
| 2020 AMC 12B (Problems • Answer Key • Resources) | |
| Preceded by Problem 15 |
Followed by Problem 17 |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
| All AMC 12 Problems and Solutions | |
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