Reply Tue 21 Mar, 2017 11:00 am
Alicia writes the number 1 to 45 on separate cards. She then randomly
chooses three of the cards. What is the probability that the 2nd and
3rd cards will include the digit 9 in the number.

Using a permutation solution method, I find the total number of
possible outcomes (45_P_3, 45 cards taken 3 at a time) which gives
85140 different arrangements of the 3 numbers.

I also use permutations to find the total number of ways the 4
numbers (9, 19, 29, and 39) can be arranged taken two at a time
(4_P_2) to get 12 different arrangements. (These are the
arrangements of the LAST 2 numbers with a 9). For each of the 12
arrangements, there are 43 different numbers that can come first
in the 3 letter group, so 12 * 43 = 516 total arrangements
(desired outcomes)

Using the basic probability model I divide the number of desired
outcomes by the number of possible outcomes.

516/85140 = .00606 = .606%

*I have even made lists to check that this is correct.*

However, the answer in the book uses the multiplication rule
(which seems valid) to find the solution.

P(any card the fist time) * P(9 the 2nd time) * P(9 the 3rd time)

=45/45 * 4/44 * 3/43

= .00634 = .634%

This doesn't agree with the permutation method (which I trust more
unless you can find a flaw).

However, I can get the same solution by multiplication if I start
with 45 in the denominator.

4/45 * 3/44 = .00606 = .606%

But I can't find any logic at all in why you would do this.
Can you shed any light on this for me?

THANKS!

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engineer
 
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Reply Tue 21 Mar, 2017 11:51 am
@canaanbowman,
Sure. The first card is a red herring. Unless you know something about it, it doesn't impact the probabilities at all. That's why your calculation

4/45 * 3/44 = .00606 = .606%

works. If you knew something about the first card, that would change up the numbers. The reason the text book is wrong on it's answer is that is assumes the result of the first card doesn't impact the next cards other than to remove a card from the deck.

45/45 * 4/44 * 3/43 is clearly wrong since it assumes that the card could be a nine, but doesn't account for that likelihood in the next terms. If you wanted to consider the first card, then you have to do it twice, once for if the first card is not a 9, the second time if it is.

= (41/45 * 4/44 * 3/43)+ (4/45 * 3/44 * 2/43) = .606%
canaanbowman
 
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Reply Tue 21 Mar, 2017 12:39 pm
@engineer,
YES. Thanks!

The addition part makes perfect sense.
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