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S1 - tree diagram question help

http://www.ocr.org.uk/Images/136156-question-paper-unit-4732-probability-and-statistics-1.pdf

Question 4i)

It says two are removed at random yet on the mark scheme the next probability branch is a fraction of 5 and not 4? can a wonderful mathmagician please explain this to me :smile:

thankyou

Reply 1

Zacken

Original post by skate-elz

thankyou

So there are originally

\displaystyle 6

discs in the bag, made up of

5

Red and

1

black.

Tina pulls out the first disk and has a probability of

\displaystyle \frac{5}{6}

of it being a red disk. If she does pull out a red disk, then there are now only

5

disks left in the bag, made up of

1

black and

4

red. If Tina pulls out another one, then she has

\displaystyle \frac{5-1}{6-1} = \frac{4}{5}

probability of it being red again.

She has a

\displaystyle \frac{1}{5}

chance of it being a black one, however. There were 6 discs, she pulled out a red one, so there are 5 discs, 4 red, 1 black left.

Now that the red branch has been dealt with, let's look at the black. Tina pulls out the first disk and has a

\displaystyle \frac{1}{6}

chance of it being black. If she does pull out a black disk, there are only 5 disks left in the bag, 0 black and 5 red. so that the chance of her pulling out another black disk is 0 and the chance of her pulling out a red disk is 1.

TLDR: 6 discs, one taken one is already given in the paper. Second disk taken out means that you have to choose from the remaining 5 discs in the bag so

\displaystyle \frac{.}{5}

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S1 - tree diagram question help

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