Difference between revisions of "2020 AMC 10B Problems/Problem 19"
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<math>4A\equiv8\pmod{9} \implies A=\boxed{\textbf{(A) }2}</math> ~quacker88 | <math>4A\equiv8\pmod{9} \implies A=\boxed{\textbf{(A) }2}</math> ~quacker88 | ||
+ | |||
+ | ==Solution 1 but easier== | ||
+ | We're looking for the amount of ways we can get <math>10</math> cards from a deck of <math>52</math>, which is represented by <math>\binom{52}{10}</math>. | ||
+ | |||
+ | <math>\binom{52}{10}=\frac{52\cdot51\cdot50\cdot49\cdot48\cdot47\cdot46\cdot45\cdot44\cdot43}{10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}</math> | ||
+ | |||
+ | And after simplifying, we get <math>26\cdot17\cdot7\cdot47\cdot46\cdot5\cdot11\cdot43</math>. | ||
+ | Now, if we examine the number <math>158A00A4AA0</math>, we can notice that it is equal to some number <math>n</math> times 10. | ||
+ | Therefore, we can divide 10 from the aforementioned expression and find the units digit, which will be <math>A</math>. | ||
+ | |||
+ | Now, after dividing ten, we will have <math>26\cdot17\cdot7\cdot47\cdot23\cdot11\cdot43</math>. | ||
+ | We can then use modulo 10 and find that the unit digit of the expression is <math>\boxed{\textbf{(A) }2}</math> | ||
+ | ~lucaswujc | ||
==Solution 2== | ==Solution 2== | ||
Line 34: | Line 47: | ||
==Solution 3== | ==Solution 3== | ||
It is not hard to check that <math>13</math> divides the number, | It is not hard to check that <math>13</math> divides the number, | ||
− | <cmath>\binom{52}{10}=\frac{52\cdot51\cdot50\cdot49\cdot48\cdot47\cdot46\cdot45\cdot44\cdot43}{10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}=26\cdot17\cdot5\cdot7\cdot47\cdot46\cdot11\cdot43</cmath> | + | <cmath>\binom{52}{10}=\frac{52\cdot51\cdot50\cdot49\cdot48\cdot47\cdot46\cdot45\cdot44\cdot43}{10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}=26\cdot17\cdot5\cdot7\cdot47\cdot46\cdot11\cdot43.</cmath> As <math>10^3\equiv-1\pmod{13}</math>, using <math>\pmod{13}</math> we have <math>13|\overline{AA0}-\overline{0A4}+\overline{8A0}-\overline{15}=110A+781</math>. Thus <math>6A+1\equiv0\pmod{13}</math>, implying <math>A\equiv2\pmod{13}</math> so the answer is <math>\boxed{\textbf{(A) }2}</math>. |
+ | |||
+ | <math>\textbf{- Emathmaster}</math> | ||
+ | |||
+ | ==Solution 4== | ||
+ | As mentioned above, <br> | ||
+ | <cmath>\binom{52}{10}=\frac{52 \cdot 51 \cdot 50 \cdot 49 \cdot 48 \cdot 47 \cdot 46 \cdot 45 \cdot 44 \cdot 43}{10 \cdot 9 \cdot 8 \cdot 7 \cdot 6 \cdot 5 \cdot 4 \cdot 3 \cdot 2 \cdot 1} = {10 \cdot 17 \cdot 13 \cdot 7 \cdot 47 \cdot 46 \cdot 11 \cdot 43} = 158A00A4AA0.</cmath> | ||
+ | We can divide both sides of <math>10 \cdot 17 \cdot 13 \cdot 7 \cdot 47 \cdot 46 \cdot 11 \cdot 43 = 158A00A4AA0</math> by 10 to obtain | ||
+ | <cmath>17 \cdot 13 \cdot 7 \cdot 47 \cdot 46 \cdot 11 \cdot 43 = 158A00A4AA,</cmath> | ||
+ | which means <math>A</math> is simply the units digit of the left-hand side. This value is | ||
+ | <cmath>7 \cdot 3 \cdot 7 \cdot 7 \cdot 6 \cdot 1 \cdot 3 \equiv \boxed{\textbf{(A) }2} \pmod{10}.</cmath> | ||
+ | ~[[User:i_equal_tan_90|i_equal_tan_90]], revised by [[User:emerald_block|emerald_block]] | ||
+ | |||
+ | ==Solution 5 (Very Factor Bashy CRT)== | ||
+ | We note that: | ||
+ | <cmath> \frac{(52)(51)(50)(49)(48)(47)(46)(45)(44)(43)}{(10)(9)(8)(7)(6)(5)(4)(3)(2)(1)} = (13)(17)(7)(47)(46)(5)(22)(43). </cmath> | ||
+ | Let <math>K=(13)(17)(7)(47)(46)(5)(22)(43)</math>. This will help us find the last two digits modulo <math>4</math> and modulo <math>25</math>. | ||
+ | It is obvious that <math>K \equiv 0 \pmod{4}</math>. Also (although this not so obvious), <cmath>K \equiv (13)(17)(7)(47)(46)(5)(22)(43)</cmath> <cmath>\equiv (13)(-8)(7)(-3)(-4)(5)(-3)(-7)</cmath> <cmath>\equiv (13)(-96)(21)(35)</cmath> <cmath>\equiv (13)(4)(-4)(10)</cmath> <cmath>\equiv (13)(-16)(10)</cmath> <cmath>\equiv (13)(9)(10)</cmath> <cmath>\equiv (117)(10)</cmath> <cmath>\equiv (-8)(10)</cmath> <cmath>\equiv 20 \pmod{25}.</cmath> Therefore, <math>K \equiv 20 \mod 100</math>. Thus <math>K=20</math>, implying that <math>\boxed{\textbf{(A) }2}</math>. | ||
+ | |||
+ | ==Solution 6== | ||
+ | As in Solution 2, we see that | ||
+ | |||
+ | <cmath>\binom{52}{10}=26\cdot17\cdot5\cdot7\cdot47\cdot46\cdot11\cdot43,</cmath> | ||
+ | |||
+ | which contains no factors of <math>3.</math> Therefore, the sum of the digits must not be a multiple of <math>3.</math> This sum is | ||
+ | |||
+ | <cmath>1+5+8+A+0+0+A+4+A+A+0=18+4A.</cmath> | ||
+ | |||
+ | It follows that <math>4A</math> cannot be a multiple of <math>3,</math> ruling out choices <math>(B)</math> and <math>(D).</math> Therefore, our possibilities are <math>A=2,4,</math> and <math>7.</math> Now, notice that <math>\binom{52}{10}</math> is divisible by <math>7.</math> Therefore, we can plug each possible value of <math>A</math> into <math>158A00A4AA0</math> and test for divisibility by <math>7.</math> Conveniently, we see that the first value, <math>A=2,</math> works. Thus, the answer is <math>\boxed{\bold{(A)} 2}.</math> (To make our argument more rigorous, we can also test divisibility by <math>7</math> for <math>A=4</math> and <math>7</math> to show that these values do not work.) | ||
+ | |||
+ | --vaporwave | ||
+ | |||
+ | ==Solution 7== | ||
+ | The total number of ways to choose <math>10</math> from <math>52</math> is | ||
+ | <math>\binom{52}{10}=\frac{52\cdot51\cdot50\cdot49\cdot48\cdot47\cdot46\cdot45\cdot44\cdot43}{10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}</math> | ||
+ | |||
+ | Using divisibility rules, we have that A is not a multiple of <math>3</math>. Then, divide this equation by 10. This implies that the new number <math>158A00A4AA0</math> is divisible by <math>2</math> but not <math>4</math>. This means that <math>A</math> is either <math>2</math> or <math>6</math>. However, <math>6</math> is a multiple of <math>3</math>, meaning <math>A</math> has to be <math>\boxed {\textbf{(A)2}}</math> | ||
+ | |||
+ | ~Arcticturn | ||
+ | |||
+ | ==Solution 8 (Very time consuming)== | ||
+ | As stated in previous solutions, the number of ways to choose <math>10</math> from <math>52</math> is | ||
+ | <math>\binom{52}{10}=\frac{52\cdot51\cdot50\cdot49\cdot48\cdot47\cdot46\cdot45\cdot44\cdot43}{10\cdot9\cdot8\cdot7\cdot6\cdot5\cdot4\cdot3\cdot2\cdot1}</math> | ||
+ | |||
+ | Canceling out common factors <math>(\dfrac{52}{2} = 26 \text{, } \dfrac{51}{3} = 17 \text{, } 5 \cdot 10 = 50 \text{, } \dfrac{49}{7} = 7 \text{, } 48 = 6 \cdot 8 \text{, } \dfrac{45}{9} = 5 \text{, } \dfrac{44}{11} = 4)</math>, you get this - | ||
+ | <math>\frac{\cancel{52}^{26}\cdot\cancel{51}^{17}\cdot\cancel{50}\cdot\cancel{49}^{7}\cdot\cancel{48}\cdot47\cdot46\cdot\cancel{45}^5\cdot\cancel{44}^{4}\cdot43}{\cancel{10}\cdot\cancel{9}\cdot\cancel{8}\cdot\cancel{7}\cdot\cancel{6}\cdot\cancel{5}\cdot\cancel{4}\cdot\cancel{3}\cdot\cancel{2}\cdot\cancel{1}}</math> | ||
+ | |||
+ | When you multiply the remaining numbers, you get the product as <math>15820024220</math>. From this product, we can then determine that <math>A</math> is equal to <math>\boxed {\textbf{(A)2}}</math> | ||
+ | |||
+ | ~ KING.OF.MATH | ||
+ | |||
+ | ~[https://artofproblemsolving.com/wiki/index.php/User: Puck_0] (Minor LaTeX) | ||
+ | |||
+ | ==Solution 9== | ||
+ | Compute <math>\frac{52!}{10!42!} = 15820024220.</math> Therefore our answer is <math>\boxed {\textbf{(A)2}}.</math> | ||
+ | |||
+ | ~ Sliced_Bread | ||
+ | |||
+ | ==Video Solutions== | ||
+ | ==Video Solution (HOW TO THINK CREATIVELY!!!)== | ||
+ | https://youtu.be/QNUzrwXWQ2A | ||
+ | |||
+ | ~Education, the Study of Everything | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | |||
− | ==Video Solution== | + | ===Video Solution=== |
https://youtu.be/3BvJeZU3T-M | https://youtu.be/3BvJeZU3T-M | ||
− | ~ | + | ===Video Solution 2=== |
+ | https://www.youtube.com/watch?v=ApqZFuuQJ18&list=PLLCzevlMcsWNcTZEaxHe8VaccrhubDOlQ&index=6 ~ MathEx | ||
==See Also== | ==See Also== |
Latest revision as of 19:58, 22 October 2023
Contents
Problem
In a certain card game, a player is dealt a hand of cards from a deck of
distinct cards. The number of distinct (unordered) hands that can be dealt to the player can be written as
. What is the digit
?
Solution 1
We're looking for the amount of ways we can get cards from a deck of
, which is represented by
.
We need to get rid of the multiples of , which will subsequently get rid of the multiples of
(if we didn't, the zeroes would mess with the equation since you can't divide by 0)
,
,
leaves us with 17.
Converting these into, we have
~quacker88
Solution 1 but easier
We're looking for the amount of ways we can get cards from a deck of
, which is represented by
.
And after simplifying, we get .
Now, if we examine the number
, we can notice that it is equal to some number
times 10.
Therefore, we can divide 10 from the aforementioned expression and find the units digit, which will be
.
Now, after dividing ten, we will have .
We can then use modulo 10 and find that the unit digit of the expression is
~lucaswujc
Solution 2
Since this number is divisible by but not
, the last
digits must be divisible by
but the last
digits cannot be divisible by
. This narrows the options down to
and
.
Also, the number cannot be divisible by . Adding up the digits, we get
. If
, then the expression equals
, a multiple of
. This would mean that the entire number would be divisible by
, which is not what we want. Therefore, the only option is
-PCChess
Solution 3
It is not hard to check that divides the number,
As
, using
we have
. Thus
, implying
so the answer is
.
Solution 4
As mentioned above,
We can divide both sides of
by 10 to obtain
which means
is simply the units digit of the left-hand side. This value is
~i_equal_tan_90, revised by emerald_block
Solution 5 (Very Factor Bashy CRT)
We note that:
Let
. This will help us find the last two digits modulo
and modulo
.
It is obvious that
. Also (although this not so obvious),
Therefore,
. Thus
, implying that
.
Solution 6
As in Solution 2, we see that
which contains no factors of Therefore, the sum of the digits must not be a multiple of
This sum is
It follows that cannot be a multiple of
ruling out choices
and
Therefore, our possibilities are
and
Now, notice that
is divisible by
Therefore, we can plug each possible value of
into
and test for divisibility by
Conveniently, we see that the first value,
works. Thus, the answer is
(To make our argument more rigorous, we can also test divisibility by
for
and
to show that these values do not work.)
--vaporwave
Solution 7
The total number of ways to choose from
is
Using divisibility rules, we have that A is not a multiple of . Then, divide this equation by 10. This implies that the new number
is divisible by
but not
. This means that
is either
or
. However,
is a multiple of
, meaning
has to be
~Arcticturn
Solution 8 (Very time consuming)
As stated in previous solutions, the number of ways to choose from
is
Canceling out common factors , you get this -
When you multiply the remaining numbers, you get the product as . From this product, we can then determine that
is equal to
~ KING.OF.MATH
~Puck_0 (Minor LaTeX)
Solution 9
Compute Therefore our answer is
~ Sliced_Bread
Video Solutions
Video Solution (HOW TO THINK CREATIVELY!!!)
~Education, the Study of Everything
Video Solution
Video Solution 2
https://www.youtube.com/watch?v=ApqZFuuQJ18&list=PLLCzevlMcsWNcTZEaxHe8VaccrhubDOlQ&index=6 ~ MathEx
See Also
2020 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 18 |
Followed by Problem 20 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.