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# S1 probability watch

1. Sorry about the webcam quality, but I need help on Q6d and Q6f

I cannot understand why the answer to Q6d is 2/20. I keep getting 1/20 because the probability of even is 10/20 and the probability of square is 4/20.
10/20 * 4/20 = 1/10

Logically thinking Q6f has to be 0 because you cant have both even and odd and this is also the answer at the back of the book. But, if you use the rule of muliplying (because its add), you get 10/20 * 10/20 = 0.25

2. Because not all evens are square. E.g. 9 is even but not square, so it's out of the probability. In this case, 4 and 16 are the only two options. Two out of 20.

Even and odd are mutually exclusive, so the probability is 0, because they have no common points, in a Venn diagram.
3. Whats wrong with the method im using though?
4. To use the 'multiplication rule' the two outcomes cannot be mutually exclusive, which in this case they are.

"10/20 and the probability of square is 4/20"

"10/20 * 4/20 = 1/10 "

Now am I just retarded or is 1/10 = 2/20

You've done it right with your technique.
5. (Original post by hassi94)
To use the 'multiplication rule' the two outcomes cannot be mutually exclusive,...
6. (Original post by nasira372)
Well yes it's just logic.

If two outcomes are mutually exclusive. The probability of them both happening is ALWAYS = zero.
7. (Original post by hassi94)
Well yes it's just logic.

If two outcomes are mutually exclusive. The probability of them both happening is ALWAYS = zero.
You sure told me....

Im sorry for ever questioning you

Still dont understand 6D tho
8. (Original post by nasira372)
You sure told me....

Im sorry for ever questioning you

Still dont understand 6D tho
You can't multiply probabilities like that for this question. You've just got to look at how many are even and squares and divide by 20, which is the total number of numbers, to get the probability.
9. (Original post by nasira372)
You sure told me....

Im sorry for ever questioning you

Still dont understand 6D tho
You can work this out explicitly as you only have 20 numbers to check.

Even numbers: 2,4,6,8,10,12,14,16,18,20
Square numbers: 1,4,9,16

How many numbers lie in both lists? What's this as a fraction out of 20? (Each number is just as likely to come up, with probability 1/20)
10. (Original post by nasira372)
You sure told me....

Im sorry for ever questioning you

Still dont understand 6D tho
I'm 100% sure

You can multiply probabilities for 6D

Probability of even is 10/20

Probability of square is 4/20

10/20 * 4/20 = 40/400 = 2/20

You did it right, but you're just not realising that your answer of 1/10 is the SAME as 2/20!
11. (Original post by hassi94)
I'm 100% sure

You did it right, but you're just not realising that your answer of 1/10 is the SAME as 2/20!
12. (Original post by nasira372)
Haha don't worry we all miss simple things sometimes :P

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Updated: April 14, 2011
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