Abracadabra!

At a recent magic show, the magician has 10 cards, each assigned a distinct integer from 1 to 10 (inclusive). After randomly shuffling the cards, he claims that they are now in order, which the audience disbelieves. Let p p be the probability that exactly 9 of the cards are arranged in order. What is the value of 1000 p \lfloor 1000p \rfloor ?

Details and assumptions

The function x : R Z \lfloor x \rfloor: \mathbb{R} \rightarrow \mathbb{Z} refers to the greatest integer smaller than or equal to x x . For example 2.3 = 2 \lfloor 2.3 \rfloor = 2 and 5 = 5 \lfloor -5 \rfloor = -5 .

The card is in order if it's position in the sequence corresponds to the numerical value of the card. As an explicit example, the sequence 1 , 3, 5, 7, 9, 10, 6, 8 , 2, 4 has 2 cards (listed in bold) that are in order.


The answer is 0.

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12 solutions

Tanishq Aggarwal
Sep 9, 2013

Theorem: It is impossible to have exactly nine cards arranged in order. Proof: For the sake of contradiction, assume that exactly nine cards are arranged in order. Then the last card must be in order as well, as it has no other place to go (or else the original nine cards are not in order). This would mean that all ten cards are in order, a contradiction to the original assumption. Thus p = 0 p=0 , and our desired answer is 0 \boxed{0} .

I think that there are arrangements of the cards that would satisfy the problem but the number is very low compared to 10! which is the number of permutations of the cards

Rindell Mabunga - 7 years, 9 months ago

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No there are no such arrangements......

Led Tasso - 7 years, 6 months ago

I noticed this but thought the problem was incorrect :/

Sebastian Garrido - 7 years, 8 months ago
David Wu
Sep 8, 2013

By "in order" I assume that a hand such as 10 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 10,1,2,3,4,5,6,7,8,9 would work - no wrap arounds First we pick 9 9 of the integers in ( 10 9 ) \dbinom{10}{9} ways. There is only one permutation of them which satisfy the conditions Then we have to pick where the last integer can go - normally, it would have 10 10 spots but one of them will violate the conditions; all 10 10 cards would be in order. So there are 9 9 spots that the last integer can go. Finally, there are 10 ! 10! ways to order the cards without regard to the restrictions. The probability p = ( 10 9 ) 9 10 ! = 1 8 ! p = \dfrac{\dbinom{10}{9} \cdot 9}{10!} = \dfrac{1}{8!} Then 1000 p = 1000 8 ! < 1 1000 p = 0 1000p = \dfrac{1000}{8!} < 1 \implies \lfloor 1000p\rfloor = \boxed{0}

Moderator note:

I've clarified what I meant by "in order", which is different from your interpretation. Sorry about the ambiguity.

can u plz xplain the 9 spots part ?

Pranay Baldev - 7 years, 9 months ago

I think that there is 3 possible arrangements out of 10! permutations of the 10 cards, which will give a nearly 0 result

Rindell Mabunga - 7 years, 9 months ago

According to my interpretation, If we have 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 1,2,3,4,5,6,7,8,9 , and we want to place the 10 10 , we can't place it last, otherwise all 10 10 cards would be in order. So it can be in any of the other 9 9 spots.

David Wu - 7 years, 9 months ago
Arnav Shringi
Sep 11, 2013

P is 0 because you cannot have just one card in the incorrect spot. The least number of cards that can be placed in the incorrect spot is two.

Now, since P=0,

So, 1000P=1000*0=

                          0

It's impossible to obtain an arrangement in which exactly nine of the ten cards are in order. Either all cards are in order, or else at least two of the cards must not be in order, leaving at most 10 2 = 8 10-2=8 cards in order.

Conner Davis
Sep 10, 2013

Assume 9 of the cards are in order. There is only one remaining place for the last card to be, but that place would put the card in order. Therefore it is impossible to have exactly 9 cards in order. Thus the probability is 0,

Kevin Fei
Sep 9, 2013

If nine of the cards are already in order, in their numerical positions, the last one has only one place to go. That position corresponds to the numerical value of the card and makes all 10 cards in order. Thus there is no way that exactly 9 are in order. p = 0 p = 0

If you start with 10 cards and exactly 9 cards are in order, then all the 10 cards are in order. So the probability of exactly 9 cards being in order is 0.

There are 10 ! 10! possible ways for the cards to be ordered. However only in two of them exactly 9 of the cards are arranged in numerical order. They are:

10, 1,2,3,4,5,6,7,8,9

and

2,3,4,5,6,78,9,10 ,1

Thus the probability that exactly 9 of the cards are arranged in numerical order would be 2 10 ! \frac{2}{10!} . Consequently:

1000 × 2 10 ! = 5 9 × 8 × 7 × 6 × 3 = 0 \lfloor{1000\times\frac{2}{10!}} \rfloor = \frac{5}{9 \times 8 \times 7 \times 6 \times 3} = 0

Moderator note:

Sorry, I was in the midst of clarifying the question. I though of using "numerical order", but then realized that it didn't convey what I wanted.

oops! a comma is missed. The second one is 2,3,4,5,6,7,8,9,10 ,1

Alireza Mirsadeghi - 7 years, 9 months ago

OK! With this clarification, there would be no arrangement in which exactly 9 cards have the same position in the list as their numerical value indicates. In other words, if 9 cards are seated in their right positions, the 10th card will inevitably be placed in the right position as well.

Alireza Mirsadeghi - 7 years, 9 months ago
Jc Ng
Sep 14, 2013

It is not possible for only 1 card to be in a wrong position. For a card to be in a wrong position, it has to occupy the supposed position of another card, and that card will in turn be displaced to another wrong position. Therefore, p=0, and 1000p=0.

Zechariah Zhu
Sep 13, 2013

If exactly 9 of the cards are arranged in order, then no matter what, the 10th card must be filled in the last spot, so all 10 cards must be arranged in order. This is a contradiction since we assumed that exactly 9 cards were arranged in order. So the probability that exactly 9 are in order is 0, and 0x1000 = 0, our answer.

Abin Das
Sep 11, 2013

If 9 cards are in order,then the 10th card will also be in order.Thus the probability that exactly 9 cards are in order is 0 .

Chengfang Goh
Sep 10, 2013

The total ways of arranging the cards=10!=3628800

The number of ways that exactly 9 of the cards are arranged in order=1x10=10

p = the probability that exactly 9 of the cards are arranged in order = The number of ways that exactly 9 of the cards are arranged in order/The total ways of arranging the cards=10/3628800=2.7557E-6

1000p=1000x2.7557E-6=2.7557E-3

The floor value of 1000p is thus 0

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