Original practice · fully worked
Original variant: conditioned archive tokens
An archive contains 6 bronze, 5 silver, and 4 gold tokens. Five tokens are selected without replacement. Given that exactly two selected tokens are bronze, calculate the probability that at least one selected token is gold.
- A
0.119 - B
0.444 - C
0.476 - D
0.881 - E
1.000
Answer and calculation
After conditioning on two bronze tokens, the other three are drawn from five silver and four gold tokens. The probability that all three are silver is 10/84, so the probability of at least one gold is 37/42=0.880952 and choice D is correct.
Setup
Setup
Conditioning fixes two bronze selections, leaving three selections from the nine nonbronze tokens.
5 silver+4 gold=9 nonbronze Model
Model
Use the complement event that all three remaining selections are silver.
Pr(at least one gold∣two bronze) =1−(39)(35) Compute
Compute
Evaluate the combination ratio.
1−8410=8474=4237 4237=0.8809523810… Answer
Answer
The conditional probability rounds to 0.881.
0.881(D)