abbie1259
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Hello, I'm really struggling with probability questions and I'd be very grateful if somebody could explain to me how to do the following question:

'Carolyn has 20 biscuits in a tin.

She has
12 plain biscuits
5 chocolate biscuits
3 ginger biscuits.

Carolyn takes at random two biscuits from the tin.

Work out the probability that the two biscuits were not the same type.'

Thank you!
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Pinixy5
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Hey,

The probability would be:

The total number of biscuits = 20

(plain, plain) = (12/20)(11/19) = 132/380
(chocolate,chocolate) = (5/20)(4/19) = 20/380
(ginger,ginger) = (3/20)(2/19) = 6/380

Probability (they both are of the same type) = 132/380+ 20/380 + 6/380 = 158/380 = 79/190

Probability (they were not the same type.) = 1- 79/190 = 111/190

I hope this helps. :-)
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abbie1259
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(Original post by Hammarobi)
Hey, i can quickly, tell you what it is, and what to revise, i'm sure someone else can tell you step by step (I would but I've gotta go, Sorry)

It's a probability tree diagram, this uses 3 lines, and has two of the diagrams as she takes 2 Biscuits.

The tree diagram is a, how i like to remember them, a webbed foot like drawing.

(I'll finish this off later if someone else doesn't come)
(Original post by Pinixy5)
Hey,

The probability would be:

The total number of biscuits = 20

(plain, plain) = (12/20)(11/19) = 132/380
(chocolate,chocolate) = (5/20)(4/19) = 20/380
(ginger,ginger) = (3/20)(2/19) = 6/380

Probability (they both are of the same type) = 132/380+ 20/380 + 6/380 = 158/380 = 79/190

Probability (they were not the same type.) = 1- 79/190 = 111/190

I hope this helps. :-)
Thank you both very much, this was very clear and helped a lot!
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reisamimi
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why was the denominator 19 when you were working it out, if the question said they take 2 biscuits
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