A probability question, who can help me out (with steps)?

2011-09-07 11:32 am

3. A box contains 4 black cubes and 6 white cubes. A cube is drawn from the box. Its colour
noted and a cube of the other colour is then added to the box. A second cube is then drawn.
(a) If both cubes are of the same colour, what is the probability that both cubes were in
fact white?
(b) The first ball is replaced before the second ball is added to the box.

please list the calculation and steps thanks

更新1:

sorry :P missed should be:What is the probability that both cubes were white given that both cubes were of the same colour?

更新2:

question one answer is 5/7 and second answer is 9/13, just for you guys to refer, thanks

回答 (2)

用戶42622

2011-09-07 11:44 am

✔ 最佳答案

the probability of drawing two black boxes

you have a 4/10 probability of drawing the first box
then you have a 3/10 probability of drawing the second black box

(4/10)(3/10) = 12/100 probability that both boxes are black

white boxes
(6/10)(5/10) = 30/100 probability that both boxes are white

you have a
(12 + 30) / 100 = 42/100 probability of drawing two boxes of the same color

(30 / 100) / (42 / 100)
(3/10)(50/21) = 150/210
5/7 probability of drawing two white boxes given you drew two boxes of the same color

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[匿名]

2011-09-07 6:37 pm

total number of cubes is 10.

probability of pulling out a white cube is 6/10
Probability of pulling out a second white cube again is 5/9 because one white cube was taking out (total cube numbers is decreased by one and white cube number is decreased by one)

Multiply these probabilities and you get 1/3

(b) Ball? What ball?

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