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Y11 maths h/w help! Tree Diagrams!

(22 Posts)
Draylon Mon 02-Feb-15 17:06:58

"A bag contains 10 cubes, 3 red, 7 blue. A cube is picked at random and after being replaced, another cube is picked. Draw a tree diagram to show all possible outcomes"

Now, I have a C grade GCE, 1978 (2nd go!) and bitesize to help grin, so am a bit clueless.

I have a tiny idea but I'm not at all sure. And I recognise that it's hard in a text box to show me... but can anyone help?

TIA

18yearstooold Mon 02-Feb-15 17:08:48

Am I missing something?

There's only 2 potential outcomes, red or blue?

Draylon Mon 02-Feb-15 17:11:06

I can't get beyond there being a 70% chance it'll be blue and a 30% chance it'll be red confused. And every go, the odds remain the same!

I think I'm missing something, too!

Camolips Mon 02-Feb-15 17:11:57

Are you sure you put the cube back in the bag?

penny13610 Mon 02-Feb-15 17:14:03

page 182 www.cimt.plymouth.ac.uk/projects/mepres/allgcse/bka5.pdf

whattodoforthebest2 Mon 02-Feb-15 17:14:19

www.mathsisfun.com/data/probability-tree-diagrams.html

Instead of head or tail in the above illustration, you have red or blue, there is a 0.3 chance of red, 0.7 chance of blue. Then extend the tree to 4 further branches, red and blue under the initial red branch, and red and blue under the initial blue branch. Each red is 0.3 and each blue is 0.7. Then you're multiplying the probabilities, ie 0.3 x 0.3; 0.7 x 0.3; etc. Difficult without drawing lines in felt pen over my laptop screen!!

Any help?

drwitch Mon 02-Feb-15 17:14:50

Hard to explain without drawing it but I will have a go
you start off with two branches representing the outcomes that can happen when you pick the first cube. then you branch off from each of the two branches to give you the outcomes that can happen when you pick the second cube
so possible outcomes are Blue Blue , Blue Red, Red Blue and Red Red

whattodoforthebest2 Mon 02-Feb-15 17:16:04

What you're working out is the probability of red and then red, blue and then red, red and then blue, blue and then blue.

drwitch Mon 02-Feb-15 17:17:39

Blue Blue 0.7 x 0.7 = 0.49
Blue Red 0.7x 0.3 = 0.21
(note that 0.49+0.21=0.7)
Red, Blue = 0.3x 0.7 = 0.21 (note is the same as Red, Blue)
Red, Red = 0.3x0.3 = 0.09
(note that 0.21+0.09 = 0.3)

Camolips Mon 02-Feb-15 17:23:03

I'm with it now. They only want the outcomes of picking from the bag twice.

Draylon Mon 02-Feb-15 17:33:08

OK, let me get my 'ead around this a mo! And thanks!

Draylon Mon 02-Feb-15 17:44:56

This, distilled in a nutshell, is what pee's me off with GCSE.

Why did they not ask: "A bag contains 10 cubes, 3 red, 7 blue. A cube is picked at random and after being replaced, another cube is picked. Draw a tree diagram to show what the probabilities of picking red/red; blue/blue; red/blue; and blue/red are".

THEN I'd get that it is just picking out of the bag twice! (Bet the IGCSE would've phrased it my way! grin)

Thanks again, all!

TeenAndTween Mon 02-Feb-15 18:30:00

Further info, (if it just confuses then feel free to ignore.)

General rule is you multiply along the branches.

If you add up all the end possibilities it needs to total 1, otherwise something's gone wrong.

I think it is often easier to do with fractions, e.g. 3/10 x 3/10 (should be able to multiply fractions at GCSE level).

If the cube was not being replaced then probabilities change.
So after picking a red cube, you now have a 2/9 chance of another red, or 7/9 chance of picking a blue.

Chances of something not happening are 1 - chance of it happening.

So chance of 3 heads in a row is 1/8.
Chances of not getting 3 heads in a row is 1 - 1/8 = 7/8.

Draylon Mon 02-Feb-15 18:42:44

teen is your head the size of a planet or is mine micro?! grin

I am working my way through your post right now! And thanks.

TeenAndTween Mon 02-Feb-15 18:46:31

Draylon grin .
No, but I do have both a maths degree, and a DD in y11 who needs a lot of maths help.

Draylon Mon 02-Feb-15 18:52:42

Maths degree? Planet head, soz! grin

TeenAndTween Mon 02-Feb-15 18:58:08

(better not tell you where from!)

Draylon Tue 03-Feb-15 08:40:50

Portsmouth Poly? grin

PastSellByDate Thu 05-Feb-15 10:22:06

Hi Draylon:

I was looking at Mr Barton's maths blog and his list of websites to help DD1 (Y7) with something - but came across this:

www.mrbartonmaths.com/goodsites.htm - a list of various sites to help GCSE students.

In there there is a link to maths casts - which is a series of quick videos explaining maths concepts:

There is a list of videos here: https://www.sites.google.com/site/mathscasts/mathscast-videos

including 2 videos on tree diagrams:

https://www.youtube.com/watch?v=4lvkf53g16A

https://www.youtube.com/watch?v=Vph0Z0Xy6r8

--------

there also are lots of other revision websites listed on Mr Barton Maths which may help too.

HTH

OhYouBadBadKitten Thu 05-Feb-15 10:36:13

Here you go smile

Draylon Thu 05-Feb-15 13:54:14

Thank you so much for going to that much trouble! We are 'the wiser'!

OhYouBadBadKitten Thu 05-Feb-15 14:09:58

not at all smile ipads are very handy!

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