Answer:
a) 0.73684
b) 2/3
Step-by-step explanation:
part a)
![P ( A is 1 / exactly two balls are 1) = \frac{P ( A is 1 and that exactly two balls are 1)}{P (Exactly two balls are one)}](https://tex.z-dn.net/?f=P%20%28%20A%20is%201%20%2F%20exactly%20two%20balls%20are%201%29%20%3D%20%5Cfrac%7BP%20%28%20A%20is%201%20and%20that%20exactly%20two%20balls%20are%201%29%7D%7BP%20%28Exactly%20two%20balls%20are%20one%29%7D)
Using conditional probability as above:
(A,B,C)
Cases for numerator when:
P( A is 1 and exactly two balls are 1) = P( 1, not 1, 1) + P(1, 1, not 1)
= ![(\frac{1}{6}* \frac{11}{12}*\frac{1}{4}) + (\frac{1}{6}*\frac{1}{12}*\frac{3}{4}) = 0.048611111](https://tex.z-dn.net/?f=%28%5Cfrac%7B1%7D%7B6%7D%2A%20%5Cfrac%7B11%7D%7B12%7D%2A%5Cfrac%7B1%7D%7B4%7D%29%20%20%2B%20%28%5Cfrac%7B1%7D%7B6%7D%2A%5Cfrac%7B1%7D%7B12%7D%2A%5Cfrac%7B3%7D%7B4%7D%29%20%3D%200.048611111)
Cases for denominator when:
P( Exactly two balls are 1) = P( 1, not 1, 1) + P(1, 1, not 1) + P(not 1, 1 , 1)
![= (\frac{1}{6}* \frac{11}{12}*\frac{1}{4}) + (\frac{1}{6}*\frac{1}{12}*\frac{3}{4}) + (\frac{5}{6}*\frac{1}{12}*\frac{1}{4})= 0.0659722222](https://tex.z-dn.net/?f=%3D%20%28%5Cfrac%7B1%7D%7B6%7D%2A%20%5Cfrac%7B11%7D%7B12%7D%2A%5Cfrac%7B1%7D%7B4%7D%29%20%20%2B%20%28%5Cfrac%7B1%7D%7B6%7D%2A%5Cfrac%7B1%7D%7B12%7D%2A%5Cfrac%7B3%7D%7B4%7D%29%20%2B%20%28%5Cfrac%7B5%7D%7B6%7D%2A%5Cfrac%7B1%7D%7B12%7D%2A%5Cfrac%7B1%7D%7B4%7D%29%3D%200.0659722222)
Hence,
![P ( A is 1 / exactly two balls are 1) = \frac{P ( A is 1 and that exactly two balls are 1)}{P (Exactly two balls are one)} = \frac{0.048611111}{0.06597222} \\\\= 0.73684](https://tex.z-dn.net/?f=P%20%28%20A%20is%201%20%2F%20exactly%20two%20balls%20are%201%29%20%3D%20%5Cfrac%7BP%20%28%20A%20is%201%20and%20that%20exactly%20two%20balls%20are%201%29%7D%7BP%20%28Exactly%20two%20balls%20are%20one%29%7D%20%3D%20%5Cfrac%7B0.048611111%7D%7B0.06597222%7D%20%5C%5C%5C%5C%3D%200.73684)
Part b
![P ( B = 12 / A+B+C = 21) = \frac{P ( B = 12 and A+B+C = 21)}{P (A+B+C = 21)}](https://tex.z-dn.net/?f=P%20%28%20B%20%3D%2012%20%2F%20A%2BB%2BC%20%3D%2021%29%20%3D%20%5Cfrac%7BP%20%28%20B%20%3D%2012%20and%20A%2BB%2BC%20%3D%2021%29%7D%7BP%20%28A%2BB%2BC%20%3D%2021%29%7D)
Cases for denominator when:
P ( A + B + C = 21) = P(5,12,4) + P(6,11,4) + P(6,12,3)
![= 3*P(5,12,4 ) =3* \frac{1}{6}*\frac{1}{12}*\frac{1}{4}\\\\= \frac{1}{96}](https://tex.z-dn.net/?f=%3D%203%2AP%285%2C12%2C4%20%29%20%3D3%2A%20%5Cfrac%7B1%7D%7B6%7D%2A%5Cfrac%7B1%7D%7B12%7D%2A%5Cfrac%7B1%7D%7B4%7D%5C%5C%5C%5C%3D%20%5Cfrac%7B1%7D%7B96%7D)
Cases for numerator when:
P (B = 12 & A + B + C = 21) = P(5,12,4) + P(6,12,3)
![= 2*P(5,12,4 ) =2* \frac{1}{6}*\frac{1}{12}*\frac{1}{4}\\\\= \frac{1}{144}](https://tex.z-dn.net/?f=%3D%202%2AP%285%2C12%2C4%20%29%20%3D2%2A%20%5Cfrac%7B1%7D%7B6%7D%2A%5Cfrac%7B1%7D%7B12%7D%2A%5Cfrac%7B1%7D%7B4%7D%5C%5C%5C%5C%3D%20%5Cfrac%7B1%7D%7B144%7D)
Hence,
![P ( B = 12 / A+B+C = 21) = \frac{\frac{1}{144} }{\frac{1}{96} }\\\\= \frac{2}{3}](https://tex.z-dn.net/?f=P%20%28%20B%20%3D%2012%20%2F%20A%2BB%2BC%20%3D%2021%29%20%3D%20%5Cfrac%7B%5Cfrac%7B1%7D%7B144%7D%20%7D%7B%5Cfrac%7B1%7D%7B96%7D%20%7D%5C%5C%5C%5C%3D%20%5Cfrac%7B2%7D%7B3%7D)