Olympiad problems

2016 NIMO Problems, 4

Tags: number theory , algebra , Dice , NIMO , probability , Quadratic

A fair 100-sided die is rolled twice, giving the numbers $a$ and $b$ in that order. If the probability that $a^2-4b$ is a perfect square is $\frac{m}{n}$, where $m$ and $n$ are relatively prime positive integers, compute $100m+n$. [i] Proposed by Justin Stevens [/i]

2016 NIMO Problems, 2

Tags: function , algebra , NIMO

For real numbers $x$ and $y$, define \[\nabla(x,y)=x-\dfrac1y.\] If \[\underbrace{\nabla(2, \nabla(2, \nabla(2, \ldots \nabla(2,\nabla(2, 2)) \ldots)))}_{2016 \,\nabla\text{s}} = \dfrac{m}{n}\] for relatively prime positive integers $m$, $n$, compute $100m + n$. [i] Proposed by David Altizio [/i]

2016 NIMO Summer Contest, 12

Tags: NIMO , summer contest , 2016 , number theory

Let $p$ be a prime. It is given that there exists a unique nonconstant function $\chi:\{1,2,\ldots, p-1\}\to\{-1,1\}$ such that $\chi(1) = 1$ and $\chi(mn) = \chi(m)\chi(n)$ for all $m, n \not\equiv 0 \pmod p$ (here the product $mn$ is taken mod $p$). For how many positive primes $p$ less than $100$ is it true that \[\sum_{a=1}^{p-1}a^{\chi(a)}\equiv 0\pmod p?\] Here as usual $a^{-1}$ denotes multiplicative inverse. [i]Proposed by David Altizio[/i]

2015 NIMO Summer Contest, 9

Tags: NIMO , summer contest , probability , expected value

On a blackboard lies $50$ magnets in a line numbered from $1$ to $50$, with different magnets containing different numbers. David walks up to the blackboard and rearranges the magnets into some arbitrary order. He then writes underneath each pair of consecutive magnets the positive difference between the numbers on the magnets. If the expected number of times he writes the number $1$ can be written in the form $\tfrac mn$ for relatively prime positive integers $m$ and $n$, compute $100m+n$. [i] Proposed by David Altizio [/i]

2014 NIMO Problems, 4

Tags: probability , NIMO , number theory , relatively prime

A black bishop and a white king are placed randomly on a $2000 \times 2000$ chessboard (in distinct squares). Let $p$ be the probability that the bishop attacks the king (that is, the bishop and king lie on some common diagonal of the board). Then $p$ can be expressed in the form $\tfrac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Compute $m$. [i]Proposed by Ahaan Rungta[/i]

2016 NIMO Problems, 1

Tags: remainder , number theory , NIMO , divisor

Let $m$ be a positive integer less than $2015$. Suppose that the remainder when $2015$ is divided by $m$ is $n$. Compute the largest possible value of $n$. [i] Proposed by Michael Ren [/i]

2015 NIMO Problems, 4

Tags: NIMO , algebra

Let $A_0A_1 \dots A_{11}$ be a regular $12$-gon inscribed in a circle with diameter $1$. For how many subsets $S \subseteq \{1,\dots,11\}$ is the product \[ \prod_{s \in S} A_0A_s \] equal to a rational number? (The empty product is declared to be $1$.) [i]Proposed by Evan Chen[/i]

2015 NIMO Summer Contest, 7

Tags: summer contest , NIMO , knight , combinatorics , chess , puzzle

The NIMO problem writers have invented a new chess piece called the [i]Oriented Knight[/i]. This new chess piece has a limited number of moves: it can either move two squares to the right and one square upward or two squares upward and one square to the right. How many ways can the knight move from the bottom-left square to the top-right square of a $16\times 16$ chess board? [i] Proposed by Tony Kim and David Altizio [/i]

2012 NIMO Summer Contest, 13

Tags: NIMO , induction

For the NEMO, Kevin needs to compute the product \[ 9 \times 99 \times 999 \times \cdots \times 999999999. \] Kevin takes exactly $ab$ seconds to multiply an $a$-digit integer by a $b$-digit integer. Compute the minimum number of seconds necessary for Kevin to evaluate the expression together by performing eight such multiplications. [i]Proposed by Evan Chen[/i]

2016 NIMO Summer Contest, 14

Tags: NIMO , summer , 2016 , number theory

Find the smallest positive integer $n$ such that $n^2+4$ has at least four distinct prime factors. [i]Proposed by Michael Tang[/i]

2012 NIMO Problems, 12

Tags: NIMO

The NEMO (National Electronic Math Olympiad) is similar to the NIMO Summer Contest, in that there are fifteen problems, each worth a set number of points. However, the NEMO is weighted using Fibonacci numbers; that is, the $n^{\text{th}}$ problem is worth $F_n$ points, where $F_1 = F_2 = 1$ and $F_n = F_{n-1} + F_{n-2}$ for $n \ge 3$. The two problem writers are fair people, so they make sure that each of them is responsible for problems worth an equal number of total points. Compute the number of ways problem writing assignments can be distributed between the two writers. [i]Proposed by Lewis Chen[/i]

2015 NIMO Summer Contest, 1

Tags: NIMO , function , summer contest

For all real numbers $a$ and $b$, let \[a\Join b=\dfrac{a+b}{a-b}.\] Compute $1008\Join 1007$. [i] Proposed by David Altizio [/i]

2015 NIMO Summer Contest, 8

Tags: prime , number theory , exponent , large number , summer contest , NIMO

It is given that the number $4^{11}+1$ is divisible by some prime greater than $1000$. Determine this prime. [i] Proposed by David Altizio [/i]

2013 NIMO Problems, 3

Tags: NIMO , probability , expected value

Richard has a four infinitely large piles of coins: a pile of pennies (worth 1 cent each), a pile of nickels (5 cents), a pile of dimes (10 cents), and a pile of quarters (25 cents). He chooses one pile at random and takes one coin from that pile. Richard then repeats this process until the sum of the values of the coins he has taken is an integer number of dollars. (One dollar is 100 cents.) What is the expected value of this final sum of money, in cents? [i]Proposed by Lewis Chen[/i]

2016 NIMO Problems, 2

Tags: NIMO , combinatorics

Sitting at a desk, Alice writes a nonnegative integer $N$ on a piece of paper, with $N \le 10^{10}$. Interestingly, Celia, sitting opposite Alice at the desk, is able to properly read the number upside-down and gets the same number $N$, without any leading zeros. (Note that the digits 2, 3, 4, 5, and 7 will not be read properly when turned upside-down.) Find the number of possible values of $N$. [i]Proposed by Yannick Yao[/i]

2012 NIMO Problems, 3

Tags: modular arithmetic , NIMO

The expression $\circ \ 1\ \circ \ 2 \ \circ 3 \ \circ \dots \circ \ 2012$ is written on a blackboard. Catherine places a $+$ sign or a $-$ sign into each blank. She then evaluates the expression, and finds the remainder when it is divided by 2012. How many possible values are there for this remainder? [i]Proposed by Aaron Lin[/i]

2017 NIMO Problems, 4

Tags: NIMO , 2016 , number theory

For how many positive integers $100 < n \le 10000$ does $\lfloor \sqrt{n-100} \rfloor$ divide $n$? [i]Proposed by Michael Tang[/i]

2014 NIMO Problems, 6

Tags: logarithms , NIMO , modular arithmetic , number theory , relatively prime , arithmetic sequence

Let $\varphi(k)$ denote the numbers of positive integers less than or equal to $k$ and relatively prime to $k$. Prove that for some positive integer $n$, \[ \varphi(2n-1) + \varphi(2n+1) < \frac{1}{1000} \varphi(2n). \][i]Proposed by Evan Chen[/i]

2012 NIMO Problems, 13

Tags: NIMO , induction

For the NEMO, Kevin needs to compute the product \[ 9 \times 99 \times 999 \times \cdots \times 999999999. \] Kevin takes exactly $ab$ seconds to multiply an $a$-digit integer by a $b$-digit integer. Compute the minimum number of seconds necessary for Kevin to evaluate the expression together by performing eight such multiplications. [i]Proposed by Evan Chen[/i]

2013 NIMO Problems, 1

Tags: NIMO

What is the maximum possible score on this contest? Recall that on the NIMO 2013 Summer Contest, problems $1$, $2$, \dots, $15$ are worth $1$, $2$, \dots, $15$ points, respectively. [i]Proposed by Evan Chen[/i]

2014 NIMO Problems, 6

Tags: algebra , polynomial , modular arithmetic , NIMO , functional equation

Let $P(x)$ be a polynomial with real coefficients such that $P(12)=20$ and \[ (x-1) \cdot P(16x)= (8x-1) \cdot P(8x) \] holds for all real numbers $x$. Compute the remainder when $P(2014)$ is divided by $1000$. [i]Proposed by Alex Gu[/i]

2015 NIMO Problems, 1

Tags: geometry , NIMO

Let $\Omega_1$ and $\Omega_2$ be two circles in the plane. Suppose the common external tangent to $\Omega_1$ and $\Omega_2$ has length $2017$ while their common internal tangent has length $2009$. Find the product of the radii of $\Omega_1$ and $\Omega_2$. [i]Proposed by David Altizio[/i]

2015 NIMO Summer Contest, 2

Tags: Sum , NIMO , summer contest

On a 30 question test, Question 1 is worth one point, Question 2 is worth two points, and so on up to Question 30. David takes the test and afterward finds out he answered nine of the questions incorrectly. However, he was not told which nine were incorrect. What is the highest possible score he could have attained? [i] Proposed by David Altizio [/i]

2016 NIMO Summer Contest, 11

Tags: NIMO , summer , 2016 , combinatorics

A set $S$ of positive integers is $\textit{sum-complete}$ if there are positive integers $m$ and $n$ such that an integer $a$ is the sum of the elements of some nonempty subset of $S$ if and only if $m \le a \le n$. Let $S$ be a sum-complete set such that $\{1, 3\} \subset S$ and $|S| = 8$. Find the greatest possible value of the sum of the elements of $S$. [i]Proposed by Michael Tang[/i]

2012 NIMO Summer Contest, 12

Tags: NIMO

The NEMO (National Electronic Math Olympiad) is similar to the NIMO Summer Contest, in that there are fifteen problems, each worth a set number of points. However, the NEMO is weighted using Fibonacci numbers; that is, the $n^{\text{th}}$ problem is worth $F_n$ points, where $F_1 = F_2 = 1$ and $F_n = F_{n-1} + F_{n-2}$ for $n \ge 3$. The two problem writers are fair people, so they make sure that each of them is responsible for problems worth an equal number of total points. Compute the number of ways problem writing assignments can be distributed between the two writers. [i]Proposed by Lewis Chen[/i]