[u]Round 5[/u]

[b]p13.[/b] In coordinate space, a lattice point is a point all of whose coordinates are integers. The lattice points $(x, y, z)$ in three-dimensional space satisfying $0 \le x, y, z \le 5$ are colored in n colors such that any two points that are $\sqrt3$ units apart have different colors. Determine the minimum possible value of $n$.


[b]p14.[/b] Determine the number of ways to express $121$ as a sum of strictly increasing positive Fibonacci numbers.


[b]p15.[/b] Let $ABCD$ be a rectangle with $AB = 7$ and $BC = 15$. Equilateral triangles $ABP$, $BCQ$, $CDR$, and $DAS$ are constructed outside the rectangle. Compute the area of quadrilateral $P QRS$.


[u] Round 6[/u]

Each of the three problems in this round depends on the answer to one of the other problems. There is only one set of correct answers to these problems; however, each problem will be scored independently, regardless of whether the answers to the other problems are correct.

[b]p16.[/b] Let $C$ be the answer to problem $18$. Suppose that $x$ and $y$ are real numbers with $y > 0$ and
$$x + y = C$$
$$x +\frac{1}{y} = -2.$$
Compute $y +\frac{1}{y}$.


[b]p17.[/b] Let $A$ be the answer to problem $16$. Let $P QR$ be a triangle with $\angle P QR = 90^o$, and let $X$ be the foot of the perpendicular from point $Q$ to segment $P R$. Given that $QX = A$, determine the minimum possible area of triangle $PQR$.


[b]p18.[/b] Let $B$ be the answer to problem $17$ and let $K = 36B$. Alice, Betty, and Charlize are identical triplets, only distinguishable by their hats. Every day, two of them decide to exchange hats. Given that they each have their own hat today, compute the probability that Alice will have her own hat in $K$ days.


[u]Round 7[/u]


[b]p19.[/b] Find the number of positive integers a such that all roots of $x^2 + ax + 100$ are real and the sum of their squares is at most $2013$.


[b]p20.[/b]  Determine all values of $k$ such that the system of equations
$$y = x^2 - kx + 1$$
$$x = y^2 - ky + 1$$
has a real solution.


[b]p21.[/b] Determine the minimum number of cuts needed to divide an $11 \times 5 \times 3$ block of chocolate into $1\times 1\times 1$ pieces. (When a block is broken into pieces, it is permitted to rotate some of the pieces, stack some of the pieces, and break any set of pieces along a vertical plane simultaneously.)


[u]Round 8[/u]


[b]p22.[/b] A sequence that contains the numbers $1, 2, 3, ... , n$ exactly once each is said to be a permutation of length $n$. A permutation $w_1w_2w_3... w_n$ is said to be sad if there are indices $i < j < k$ such that $w_j > w_k$ and $w_j > w_i$. For example, the permutation $3142756$ is sad because $7 > 6$ and $7 > 1$. Compute the number of permutations of length $11$ that are not sad.


[b]p23.[/b] Let $ABC$ be a triangle with $AB = 39$, $BC = 56$, and $CA = 35$. Compute $\angle CAB - \angle ABC$ in degrees.


[b]p24.[/b] On a strange planet, there are $n$ cities. Between any pair of cities, there can either be a one-way road, two one-way roads in different directions, or no road at all. Every city has a name, and at the source of every one-way road, there is a signpost with the name of the destination city. In addition, the one-way roads only intersect at cities, but there can be bridges to prevent intersections at non-cities. Fresh Mann has been abducted by one of the aliens, but Sophy Moore knows that he is in Rome, a city that has no roads leading out of it. Also, there is a direct one-way road leading from each other city to Rome. However, Rome is the secret police’s name for the so-described city; its official name, the name appearing on the labels of the one-way roads, is unknown to Sophy Moore. Sophy Moore is currently in Athens and she wants to head to Rome in order to rescue Fresh Mann, but she does not know the value of $n$. Assuming that she tries to minimize the number of roads on which she needs to travel, determine the maximum possible number of roads that she could be forced to travel in order to find Rome. Express your answer as a function of $n$.


PS. You should use hide for answers. Rounds 1-4 have been posted [url=https://artofproblemsolving.com/community/c4h2809419p24782489]here[/url]. Collected [url=https://artofproblemsolving.com/community/c5h2760506p24143309]here[/url].
To solve for \(\angle CAB - \angle ABC\) in the triangle \(ABC\) with sides \(AB = 39\), \(BC = 56\), and \(CA = 35\), we will use the Law of Cosines and trigonometric identities.

1. **Calculate \(\cos A\) using the Law of Cosines:**
   \[
   BC^2 = AB^2 + AC^2 - 2 \cdot AB \cdot AC \cdot \cos A
   \]
   Substituting the given values:
   \[
   56^2 = 39^2 + 35^2 - 2 \cdot 39 \cdot 35 \cdot \cos A
   \]
   \[
   3136 = 1521 + 1225 - 2730 \cdot \cos A
   \]
   \[
   3136 = 2746 - 2730 \cdot \cos A
   \]
   \[
   2730 \cdot \cos A = 2746 - 3136
   \]
   \[
   2730 \cdot \cos A = -390
   \]
   \[
   \cos A = \frac{-390}{2730} = \frac{-1}{7}
   \]

2. **Calculate \(\sin A\) using the Pythagorean identity:**
   \[
   \sin^2 A + \cos^2 A = 1
   \]
   \[
   \sin^2 A + \left(\frac{-1}{7}\right)^2 = 1
   \]
   \[
   \sin^2 A + \frac{1}{49} = 1
   \]
   \[
   \sin^2 A = 1 - \frac{1}{49} = \frac{48}{49}
   \]
   \[
   \sin A = \sqrt{\frac{48}{49}} = \frac{4\sqrt{3}}{7}
   \]

3. **Calculate \(\cos B\) using the Law of Cosines:**
   \[
   AC^2 = AB^2 + BC^2 - 2 \cdot AB \cdot BC \cdot \cos B
   \]
   Substituting the given values:
   \[
   35^2 = 39^2 + 56^2 - 2 \cdot 39 \cdot 56 \cdot \cos B
   \]
   \[
   1225 = 1521 + 3136 - 4368 \cdot \cos B
   \]
   \[
   1225 = 4657 - 4368 \cdot \cos B
   \]
   \[
   4368 \cdot \cos B = 4657 - 1225
   \]
   \[
   4368 \cdot \cos B = 3432
   \]
   \[
   \cos B = \frac{3432}{4368} = \frac{11}{14}
   \]

4. **Calculate \(\sin B\) using the Pythagorean identity:**
   \[
   \sin^2 B + \cos^2 B = 1
   \]
   \[
   \sin^2 B + \left(\frac{11}{14}\right)^2 = 1
   \]
   \[
   \sin^2 B + \frac{121}{196} = 1
   \]
   \[
   \sin^2 B = 1 - \frac{121}{196} = \frac{75}{196}
   \]
   \[
   \sin B = \sqrt{\frac{75}{196}} = \frac{5\sqrt{3}}{14}
   \]

5. **Calculate \(\sin(A - B)\) using the angle difference identity:**
   \[
   \sin(A - B) = \sin A \cos B - \cos A \sin B
   \]
   Substituting the values:
   \[
   \sin(A - B) = \frac{4\sqrt{3}}{7} \cdot \frac{11}{14} - \frac{-1}{7} \cdot \frac{5\sqrt{3}}{14}
   \]
   \[
   \sin(A - B) = \frac{44\sqrt{3}}{98} + \frac{5\sqrt{3}}{98}
   \]
   \[
   \sin(A - B) = \frac{49\sqrt{3}}{98} = \frac{\sqrt{3}}{2}
   \]
   \[
   \sin(A - B) = \sin 60^\circ
   \]
   Therefore, 
   \[
   A - B = 60^\circ \text{ or } 180^\circ - 60^\circ = 120^\circ
   \]

6. **Determine the correct angle difference:**
   Since \(\cos(A - B) > 0\), we have:
   \[
   \cos(A - B) = \cos A \cos B + \sin A \sin B
   \]
   \[
   \cos(A - B) = \frac{-1}{7} \cdot \frac{11}{14} + \frac{4\sqrt{3}}{7} \cdot \frac{5\sqrt{3}}{14}
   \]
   \[
   \cos(A - B) = \frac{-11}{98} + \frac{60}{98} = \frac{49}{98} = \frac{1}{2}
   \]
   \[
   \cos(A - B) = \cos 60^\circ
   \]
   Since \(\cos(A - B) = \frac{1}{2}\), we conclude:
   \[
   A - B = 60^\circ
   \]

The final answer is \(\boxed{60^\circ}\).