Difference between revisions of "2014 UNC) Math Contest II Problems"
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You may write answers in terms of the Fibonacci numbers <math>F_n</math>. | You may write answers in terms of the Fibonacci numbers <math>F_n</math>. | ||
| − | The Fibonacci numbers are <math>F_1 = 1, F_2 = 1, F_3 = 2, F_4 = 3, F_5 = 5, F_6 = 8, \ldots</math> | + | The Fibonacci numbers are <math>F_1 = 1, F_2 = 1, F_3 = 2, F_4 = 3, F_5 = 5, F_6 = 8, \ldots </math> |
| + | |||
They are defined by the equations <math>F1 = F2 = 1</math> and, for <math>n > 2, Fn = Fn-1 + Fn-2.</math> | They are defined by the equations <math>F1 = F2 = 1</math> and, for <math>n > 2, Fn = Fn-1 + Fn-2.</math> | ||
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==Problem 2== | ==Problem 2== | ||
| − | Define the Cheshire Cat function <math>:)</math> by | + | Define the Cheshire Cat function <math>\fbox{:)}</math> by |
| − | < | + | |
| − | :)(x) & = x \quad \text{ if }x \text{ is odd}</ | + | <cmath>\begin{align*} |
| + | \fbox{:)}(x) &= -x \quad \text {if } x \text{ is even and} \\ | ||
| + | \fbox{:)}(x) &= x \quad \text{ if }x \text{ is odd} | ||
| + | \end{align*}</cmath> | ||
| − | Find the sum < | + | Find the sum <math>\fbox{:)}(1) + \fbox{:)}(2) + \fbox{:)}(3) + \fbox{:)}(4) + . . .+ \fbox{:)}(289)</math> |
[[2014 UNC Math Contest II Problems/Problem 2|Solution]] | [[2014 UNC Math Contest II Problems/Problem 2|Solution]] | ||
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==Problem 3== | ==Problem 3== | ||
| − | Find < | + | Find <math>x</math> and <math>y</math> if <math>\frac{1}{1+\frac{1}{x}}=2</math> and <math>\frac{1}{1+\frac{1}{1+\frac{1}{1+\frac{1}{y}}}}=2</math> |
[[2014 UNC Math Contest II Problems/Problem 3|Solution]] | [[2014 UNC Math Contest II Problems/Problem 3|Solution]] | ||
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==Problem 4== | ==Problem 4== | ||
| − | On the first slate, the Queen’s jurors write the number < | + | On the first slate, the Queen’s jurors write the number <math>1</math>. On the second slate they write the |
| − | numbers < | + | numbers <math>2</math> and <math>3</math>. On the third slate the jurors write <math>4, 5</math>, and <math>6</math>, and so on, writing <math>n</math> integers on |
| − | the | + | the <math>n</math>th slate. |
| − | (a) What is the largest number they write on the < | + | (a) What is the largest number they write on the <math>20</math>th slate? |
| − | (b) What is the sum of the numbers written on the < | + | (b) What is the sum of the numbers written on the <math>20</math>th slate? |
| − | (c) What is the sum of the numbers written on the < | + | (c) What is the sum of the numbers written on the <math>n</math>th slate? |
[[2014 UNC Math Contest II Problems/Problem 4|Solution]] | [[2014 UNC Math Contest II Problems/Problem 4|Solution]] | ||
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(b) Suppose that the White Rabbit builds his square cucumber | (b) Suppose that the White Rabbit builds his square cucumber | ||
frame by connecting each corner of the garden to a point a distance | frame by connecting each corner of the garden to a point a distance | ||
| − | < | + | <math>x</math> from the next corner, going clockwise, as shown in the diagram. |
Now what is the area of the region that is enclosed in the inner | Now what is the area of the region that is enclosed in the inner | ||
square frame? | square frame? | ||
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doors. She must leave no pair of adjacent doors untried. How many different sets of doors left | doors. She must leave no pair of adjacent doors untried. How many different sets of doors left | ||
untried does Alice have to choose from? | untried does Alice have to choose from? | ||
| − | For example, Alice might try doors < | + | For example, Alice might try doors <math>1</math>, <math>2</math>, and <math>4</math> and leave doors <math>3</math> and |
| − | < | + | <math>5</math> untried. There are no adjacent doors in the set of untried doors. Note: doors <math>1</math> and <math>5</math> are adjacent. |
(b) Suppose the circular room in which Alice finds herself has nine doors of nine different sizes | (b) Suppose the circular room in which Alice finds herself has nine doors of nine different sizes | ||
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wicket. All players hit the same ball. Each player hits the ball from | wicket. All players hit the same ball. Each player hits the ball from | ||
the place the previous player has left it. When the ball is hit from | the place the previous player has left it. When the ball is hit from | ||
| − | the bottom wicket, it has a < | + | the bottom wicket, it has a <math>50</math>% chance of going to the top wicket |
| − | and a < | + | and a <math>50</math>% chance of staying at the bottom wicket. When hit from |
| − | the top wicket, it has a < | + | the top wicket, it has a <math>50</math>% chance of hitting the goal post and a |
50% chance of returning to the bottom wicket. | 50% chance of returning to the bottom wicket. | ||
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==Problem 9== | ==Problem 9== | ||
| − | In the Queen’s croquet, as described in Problem < | + | In the Queen’s croquet, as described in Problem <math>8</math>, what is the probability that the ball hits the |
| − | goal post the < | + | goal post the <math>n</math>th time the ball is hit? |
[[2014 UNC Math Contest II Problems/Problem 9|Solution]] | [[2014 UNC Math Contest II Problems/Problem 9|Solution]] | ||
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==Problem 10== | ==Problem 10== | ||
| − | The March Hare invites < | + | The March Hare invites <math>11</math> guests to a tea party. He randomly |
assigns to each guest either tea or cake, but no guest receives both. | assigns to each guest either tea or cake, but no guest receives both. | ||
The guests know that the March Hare always does this, but they | The guests know that the March Hare always does this, but they | ||
never know which guests will receive tea and which will receive | never know which guests will receive tea and which will receive | ||
cake. The guests decide to play a game. Each one tries to guess | cake. The guests decide to play a game. Each one tries to guess | ||
| − | who of all </math> | + | who of all <math>11</math> guests will get cake and who will get tea. If one guest |
has more correct guesses than all the others, that guest wins. When | has more correct guesses than all the others, that guest wins. When | ||
several guests tie for the most correct guesses, then the Dormouse selects one to be the winner by | several guests tie for the most correct guesses, then the Dormouse selects one to be the winner by | ||
Revision as of 02:00, 16 October 2014
Twenty-second Annual UNC Math Contest Final Round January 25, 2014 Three hours; no electronic devices. Show your work and justify your answers. Clearer presentations will earn higher rank. We hope you enjoy thinking about these problems, but you are not expected to do them all.
You may write answers in terms of the Fibonacci numbers 
.
The Fibonacci numbers are 
They are defined by the equations 
and, for 
Contents
Problem 1
The Duchess had a child on May 1st every two years until she had five children. This year the
youngest is 
and the ages of the children are 
, and 
. Alice notices that the sum of the ages
is a perfect square: 
. How old will the youngest be the next time the sum of
the ages of the five children is a perfect square, and what is that perfect square?
Problem 2
Define the Cheshire Cat function 
by
Find the sum 
Problem 3
Find 
and 
if 
and 
Problem 4
On the first slate, the Queen’s jurors write the number . On the second slate they write the
numbers 
and 
. On the third slate the jurors write 
, and 
, and so on, writing 
integers on
the th slate.
(a) What is the largest number they write on the 
th slate?
(b) What is the sum of the numbers written on the th slate?
(c) What is the sum of the numbers written on the th slate?
Problem 5
(a) The White Rabbit has a square garden with sides of length one meter. He builds a square cucumber frame in the center by connecting each corner of the garden to the midpoint of a far side of the garden, going clockwise, as shown in the diagram. What is the area of the region that is enclosed in the inner square frame?
(b) Suppose that the White Rabbit builds his square cucumber
frame by connecting each corner of the garden to a point a distance
from the next corner, going clockwise, as shown in the diagram.
Now what is the area of the region that is enclosed in the inner
square frame?
Problem 6
(a) Alice falls down a rabbit hole and finds herself in a circular room with five doors of five
different sizes evenly spaced around the circumference. Alice tries keys in some or all of the
doors. She must leave no pair of adjacent doors untried. How many different sets of doors left
untried does Alice have to choose from?
For example, Alice might try doors , , and 
and leave doors and
untried. There are no adjacent doors in the set of untried doors. Note: doors and are adjacent.
(b) Suppose the circular room in which Alice finds herself has nine doors of nine different sizes evenly spaced around the circumference. Again, she is to try keys in some or all of the doors and must leave no pair of adjacent doors untried. Now how many different sets of doors left untried does Alice have to choose from?
Problem 7
The Caterpillar owns five different matched pairs of socks. He keeps the ten socks jumbled in random order inside a silk sack. Dressing in the dark, he selects socks, choosing randomly without replacement. If the two socks he puts on his first pair of feet are a mismatched pair and the two socks he puts on his second pair of feet are a mismatched pair, then what is the probability that the pair he selects for his third set of feet is a mismatched pair?
Problem 8
In the Queen’s croquet, a game begins with the ball at the bottom
wicket. All players hit the same ball. Each player hits the ball from
the place the previous player has left it. When the ball is hit from
the bottom wicket, it has a 
% chance of going to the top wicket
and a % chance of staying at the bottom wicket. When hit from
the top wicket, it has a % chance of hitting the goal post and a
50% chance of returning to the bottom wicket.
(a) If Alice makes the first hit and alternates hits with the Queen, what is the probability that Alice is the first player to hit the goal post with the ball?
(b) Suppose Alice, the King, and the Queen take turns hitting the ball, with Alice playing first. Now what is the probability that Alice is the first player to hit the goal post with the ball?
Problem 9
In the Queen’s croquet, as described in Problem 
, what is the probability that the ball hits the
goal post the th time the ball is hit?
Problem 10
The March Hare invites 
guests to a tea party. He randomly
assigns to each guest either tea or cake, but no guest receives both.
The guests know that the March Hare always does this, but they
never know which guests will receive tea and which will receive
cake. The guests decide to play a game. Each one tries to guess
who of all guests will get cake and who will get tea. If one guest
has more correct guesses than all the others, that guest wins. When
several guests tie for the most correct guesses, then the Dormouse selects one to be the winner by
selecting at random one of the guessers who has tied.
(a) All the guests make their guesses at random, perhaps by tossing a coin. What is the probability that Tweedledee, the last guest to arrive, is the winner?
(b) Tweedledum is the first guest to arrive. What is the probability that one or the other of Tweedledee and Tweedledum is the winner?
(c) Suppose that instead of guessing randomly, Tweedledee always makes the guess opposite to Tweedledum’s guess. If Tweedledum guesses that a guest will have tea, then Tweedledee will guess cake. If all the other guests have guessed randomly, what is the probability that one or the other of Tweedledee and Tweedledum is the winner? Your answer should be an explicit number, but partial credit may be given for reasonable formulae.
