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**matgarf****Member**- Registered: 2005-09-10
- Posts: 1

Hi, I am doing problem set 15.3 in Tobin's Mathematics Standard Level, 3rd ed. I am having a problem with a few of the conditional probabilities.

For example, on page 518, #10:

The probability that an animal will still be alive in 12 years is 0.55 and the probability that its mate will still be alive in 12 years is 0.60. Part (d) asks for the probability that the mate is still alive in 12 years given that only one is still alive in 12 years.

Assuming that these are independent events (the solutions to the other parts support this assumption), shouldn't the probability just be 0.60, since they are independent events? It shouldn't matter what happens, the probability should remail the same, right? Where is my reasoning incorrect. The answer given in the book is 0.551. Could someone please help me out here, it is driving me crazy! My only idea is that they are not independent events, but the other answers given imply that they are independent events.

Thanks,

matgarf

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**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,552

Google "monty hall problem". It might give you some insight into this. At least that's my guess.

Also, if I was to hasard a guess, I would guess .6 / (.55 + .6) which is 0.5217 which is not your answer, oh well.

*Last edited by John E. Franklin (2005-09-13 01:32:05)*

**igloo** **myrtilles** **fourmis**

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**John E. Franklin****Member**- Registered: 2005-08-29
- Posts: 3,552

I figured it out. The answer is 27/(22+27). 22 comes from .55 times .4,

and 27 comes from .6 times .45

Pretty cool, huh?

**igloo** **myrtilles** **fourmis**

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