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PostPosted: Fri Jul 11, 2014 6:03 am 
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FPTP Numerical reasoning, book2, Q3

d. What is the probability of picking one black ball and one white ball on the first two rounds, before both are replaced ?

Total no of balls = 12, Black balls = 6, white balls = 4, spotted balls = 2.

Answer: Probability of picking a black ball = 6/12, Probability of picking a white ball = 4/11, two rounds hence - 6/12*5/11*2


DD says, what if i pick a white ball before black ball or what if I pick two balls at one time ? They haven't given a sequence i.e pick black ball before white.
At the moment her understanding of 'and' is picking simultaneously. Is there any default rule ?

Finally, how likely are these type of questions for CEM exam ?


Last edited by berks_mum on Fri Jul 11, 2014 3:26 pm, edited 1 time in total.

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PostPosted: Fri Jul 11, 2014 7:48 am 
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6 + 4 + 4 is not 12

This should not be on an 11+ paper in my opinion ...


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PostPosted: Fri Jul 11, 2014 3:25 pm 
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sorry, typo, it should be spotted = 2.


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PostPosted: Fri Jul 11, 2014 3:40 pm 
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Quote:
d. What is the probability of picking one black ball and one white ball on the first two rounds, before both are replaced ?

Total no of balls = 12, Black balls = 6, white balls = 4, spotted balls = 2.


Black then white = 6/12 x 4/11

white then black = 4/12 x 6/11

Both probabilities are the same (ie order doesn't matter) so total prob is doubled (two added together) because of the two ways.

This is more like a GCSE question imho.


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PostPosted: Mon Jul 14, 2014 9:41 am 
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Same book, FPTP Numerical reasoning, Book2, Test 5,

Q3. Theo rolls two fair dice consecutively and records the results.

c) What is the probability that the sum of the numbers displayed on the two dice is 10 ?

Answer: 36 different combinations of numbers can be achieved by rolling the two dice as 6x6 = 36. The two dice will add up to 10 in three combinations : 5/5, 4/6 and 6/4. The probability of the sum of the two dice being 10 is 3/36 (simplified to 1/12).

d) What is the probability of part c occurring twice in a row ?

=================================================================================================================================

Again, is combinations a part of ks2 syllabus ? Should the DCs be prepared for such questions or best ignored ?
Needless to say DD wasn't able to do this question and the next one(d) as it was based on this.


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PostPosted: Mon Jul 14, 2014 9:43 am 
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Not part of KS2 syllabus at all ..


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