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 be the probability that exactly 9 of the cards are arranged in order. What is the value of ⌊ 1 0 0 0 p ⌋ ?
Details and assumptions
The function ⌊ x ⌋ : R → Z refers to the greatest integer smaller than or equal to x . For example ⌊ 2 . 3 ⌋ = 2 and ⌊ − 5 ⌋ = − 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.
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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
I noticed this but thought the problem was incorrect :/
By "in order" I assume that a hand such as 1 0 , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 would work - no wrap arounds First we pick 9 of the integers in ( 9 1 0 ) 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 1 0 spots but one of them will violate the conditions; all 1 0 cards would be in order. So there are 9 spots that the last integer can go. Finally, there are 1 0 ! ways to order the cards without regard to the restrictions. The probability p = 1 0 ! ( 9 1 0 ) ⋅ 9 = 8 ! 1 Then 1 0 0 0 p = 8 ! 1 0 0 0 < 1 ⟹ ⌊ 1 0 0 0 p ⌋ = 0
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 ?
I think that there is 3 possible arrangements out of 10! permutations of the 10 cards, which will give a nearly 0 result
According to my interpretation, If we have 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , and we want to place the 1 0 , we can't place it last, otherwise all 1 0 cards would be in order. So it can be in any of the other 9 spots.
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 1 0 − 2 = 8 cards in order.
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,
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
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 1 0 ! 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 1 0 ! 2 . Consequently:
⌊ 1 0 0 0 × 1 0 ! 2 ⌋ = 9 × 8 × 7 × 6 × 3 5 = 0
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
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.
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.
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.
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 .
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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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 , and our desired answer is 0 .