probability question

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mentor ma
Posts: 66
Joined: Tue Apr 16, 2013 3:11 pm

probability question

Post by mentor ma »

is two fair dice are thrown, then what is the probability of getting an even number on one and an odd number in the other? this is from 'Book name: elevenplusexams book 2 numerical reasoning (It has 10 tests)' .


page number 41 question 4d - test 7:

this answer according to me should be 18/36 = 1/2 so answer at the back of the book seems wrong as it says 1/4.

according to me the successful choices are like {(2,1) {2,3), (3,2),(1,2),...}

please can someone advise me as I am not quite confused. thanks a lot for your help in advance 8)
mystery
Posts: 8927
Joined: Tue Jul 21, 2009 10:56 pm

Re: probability question

Post by mystery »

I would have thought it was 1/2 too, with 1/4 chance of both odd and 1/4 chance of both even.
gulat
Posts: 119
Joined: Wed Mar 20, 2013 2:16 pm

Re: probability question

Post by gulat »

Hi

The probablity of getting an even number of one dice = 1/2. Probability of getting odd number on the other dice = 1/2

Prob of getting even on one and odd on another = 1/2X1/2 = 1/4

I know the question is a bit misleading as the answer assumes that one of the dice only provides even scores and the other only provides odd scores.
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Re: probability question

Post by Guest55 »

It is wrong - you need to consider both ways round.

It doesn't matter what the first die gives just that the second one is the opposite ...
nyr
Posts: 1166
Joined: Thu Apr 04, 2013 11:39 am

Re: probability question

Post by nyr »

I think it's useful to view this as being analogous to an outcome of head and tail when two coins are tossed. The probability is the 1/2 because the possible outcomes are:
HT TH HH TT

nyr
Jean.Brodie
Posts: 451
Joined: Sat Jul 21, 2012 6:55 pm

Re: probability question

Post by Jean.Brodie »

gulat summarises it well.

Alternatively, if you do a table of outcomes you will find that there are 6 x 6 = 36 total outcomes with the two dice. Only 9 give you the (even, odd) outcome. So the answer is 9/36 or 1/4.
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Re: probability question

Post by Guest55 »

36 outcomes indeed but 18 fit the odd plus even:

1,2: 1,4: 1,6

2,1: 2,3: 2,5

3,2: 3,4: 3,6

4,1: 4,3: 4,5

5,2: 5,4: 5,6

6,1: 6,3: 6,5
Jean.Brodie
Posts: 451
Joined: Sat Jul 21, 2012 6:55 pm

Re: probability question

Post by Jean.Brodie »

Sometimes one needs to understand the answer on the basis that that the question is not that well worded. The book's answer is for the outcomes where the first die gives an even. That is how I understood the question on first reading it.
Guest55
Posts: 16254
Joined: Mon Feb 12, 2007 2:21 pm

Re: probability question

Post by Guest55 »

Then the question is wrong as probability always considers both options unless told specifically that the order is important.

JB - I do teach maths!
mentor ma
Posts: 66
Joined: Tue Apr 16, 2013 3:11 pm

Re: probability question

Post by mentor ma »

thanks for your help and advise, everyone. i m now convinced that correct answer of this question is 1/2. i am a master of science in maths and i was pretty sure question/answer combination is wrong. there is at least one more probability question in the same book elsewhere that is wrong. i will write to them.
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