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Question

S4 Mathematics

Question post on Thu, 23/02/2017 3:18pm

Hi all,

Can anyone help with the following question? Thanks 🙂

Box C contains 3 red balls and 4 blue balls. Box D contains 4 red balls, 4 blue balls and an unknown number of black balls. A red ball is randomly drawn. It is further known that the probability that this drawn red ball came from Box C is 0.6. Find the number of black balls in Box D.

The answer is 6.

Answer

dazzlego · Answer 1 · Post on Thu, 23/02/2017 4:07pm

dazzlego Post on Thu, 23/02/2017 4:07pm

Hope you understand the steps…

Just need to understand the logic behind it, i.e. the one I circle in red

dazzlego · Answer 2 · Post on Thu, 23/02/2017 4:07pm

Hope you understand the steps…

Just need to understand the logic behind it, i.e. the one I circle in red

arosisi · Answer 3 · Post on Thu, 23/02/2017 5:52pm

Hi dazzlego,

Thanks for the answer.

Can you elaborate why you multiply by 1/2 when finding P(R_C) and P(R_D)?

dazzlego · Answer 4 · Post on Fri, 24/02/2017 2:26pm

It’s probability of choosing one given box…

If there are 3 boxes, the probability of choosing 1 given box is 1/3.

P(Rc) = P(box C is chosen) x P(red balls in box C is taken)

arosisi · Answer 5 · Post on Fri, 24/02/2017 2:38pm

arosisi Post on Fri, 24/02/2017 2:38pm

I see. Thanks 🙂