AMC 12C 2020 Problems

Problem 1

What is the the sum of the solutions of the equation $(x + 4)(x - 5)(x + 6)(x - 8) = 0$ ?

$\textbf{(A)}\ -3 \qquad\textbf{(B)}\ 0 \qquad\textbf{(C)}\ 2 \qquad\textbf{(D)}\ 4 \qquad\textbf{(E)}\ 7$

Problem 2

What is the numerical value of the sum $\sum_{k = 1}^{11}(i^{3} + i^{2})$

$\textbf{(A)}\ 4000 \qquad\textbf{(B)}\ 4608 \qquad\textbf{(C)}\ 4862 \qquad\textbf{(D)}\ 5792 \qquad\textbf{(E)}\ 6100$

Problem 3

In a bag are $7$ marbles consisting of $3$ blue marbles and $4$ red marbles. If each marble is pulled out $1$ at a time, what is the probability that the $6th$ marble pulled out red?

$\textbf{(A)}\ 0 \qquad\textbf{(B)}\ \frac{1}{8} \qquad\textbf{(C)}\ \frac{1}{2} \qquad\textbf{(D)}\ \frac{4}{7} \qquad\textbf{(E)}\ 1$

Problem 4

$10$ cows can consume $40$ kilograms of grass in $20$ days. How many more cows are required such that all the cows together can consume $60$ kilograms of grass in $10$ days?

$\textbf{(A)}\ 20 \qquad\textbf{(B)}\ \ 21 \qquad\textbf{(C)}\ \ 22 \qquad\textbf{(D)}\ \ 23 \qquad\textbf{(E)}\ 24$

Problem 5

Lambu the Lamb is tied to a post at the origin $(0, 0)$ on the real $xy$ plane with a rope that measures $6$ units. $2$ wolves are tied with ropes of length $6$ as well, both of them being at points $(6, 6)$ , and $(-6, -6)$ . What is the area that the lamb can run around without being in the range of the wolves?

$\textbf{(A)}\ 70 \qquad\textbf{(B)}\ 71 \qquad\textbf{(C)}\ 72 \qquad\textbf{(D)}\ 100 \qquad\textbf{(E)}\ 110$

Problem 6

How many increasing(lower to higher numbered) subsets of $\{1, 2, 3, 4, 5, 6, 7, 8, 9, 10\}$ contain no $2$ consecutive prime numbers?

Problem 7

Let $T(n)$ denote the sum of the factors of a positive integer $n$ . What is the sum of the $3$ least possible values of $x$ such that $T(x) + T(2x) = 8$ ?

Problem 8

The real value of $n$ that satisfies the equation $ln(n) + ln(n^{2} - 34) = ln(72)$ can be written in the form $a + \sqrt{b}$ where $a$ and $b$ are integers. What is $a + b$ ?

$\textbf{(A)}\ 10 \qquad\textbf{(B)}\ 16 \qquad\textbf{(C)}\ 18 \qquad\textbf{(D)}\ 21 \qquad\textbf{(E)}\ 24$

Problem 9

Let $R(x)$ denote the number of trailing $0$ s in the numerical value of the expression $x!$ , for example, $R(5) = 1$ since $5! = 120$ which has $1$ trailing zero. What is the sum

$R(20) + R(19) + R(18) + R(17) + … + R(3) + R(2) + R(1) + R(0)$ ?

$\mathrm{(A) \ } 38 \qquad \mathrm{(B) \ } 42\qquad \mathrm{(C) \ } 46\qquad \mathrm{(D) \ } 50\qquad \mathrm{(E) \ } 54$

Problem 10

In how many ways can $10$ candy canes and $9$ lollipops be split between $8$ children if each child must receive atleast $1$ candy but no child receives both types?

Problem 11

Let $ABCD$ be an isosceles trapezoid with $\overline{AB}$ being parallel to $\overline{CD}$ and $\overline{AB} = 5$ , $\overline{CD} = 15$ , and $\angle ADC = 60^\circ$ . If $E$ is the intersection of $\overline{AC}$ and $\overline{BD}$ , and $\omega$ is the circumcenter of $\bigtriangleup ABC$ , what is the length of $\overline{E\omega}$ ?

$\textbf{(A)} \frac {31}{12}\sqrt{3} \qquad \textbf{(B)} \frac {35}{12}\sqrt{3} \qquad \textbf{(C)} \frac {37}{12}\sqrt{3} \qquad \textbf{(D)} \frac {39}{12}\sqrt{3} \qquad \textbf{(E)} \frac {41}{12}\sqrt{3} \qquad$

Problem 12

Rajbhog, Aditya, and Suman are racing a $1000$ meter race. Aditya beats Rajbhog by $9$ seconds and beats Suman by $250$ meters. Given that Rajbhog beat Suman by $2$ seconds, by how many meters would Aditya beat Rajbhog if they both were having a $3500$ meter race?

Problem 13

In how many ways can the first $15$ positive integers; $\{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15\}$ in red, blue, and green colors if no $3$ numbers $a, b$ , and $c$ are the same color with $a + b - c$ being even?

Problem 14

A function $f:\mathbb{C}\to\mathbb{C}$ (Function along Complex Numbers) is defined by $f(z) = (a + bi)z$ , where $a$ and $b$ are positive real numbers. The function $f(z)$ has the property that there are complex numbers $z$ such that $z, f(z)$ , and $f^{2}(z)$ form a triangle with an area of $20$ . Given that $a + b = 10$ and $(ab)^{2} = 30$ , there is a certain complex number $c + di$ where $d$ is minimized as much as possible satisfying this condition. What is $c + d$ ?

Problem 15

Let $N = 10^{10^{100…^{10000…(100 zeroes)}}}$ . What is the remainder when $N$ is divided by $629$ ?

Problem 16

For some positive integer $k$ , let $k$ satisfy the equation

$log(k - 2)! + log(k - 1)! + 2 = 2 log(k!)$ . What is the sum of the digits of $k$ ?

$\textbf{(A)}\ 2 \qquad\textbf{(B)}\ 5 \qquad\textbf{(C)}\ 8 \qquad\textbf{(D)}\ 14 \qquad\textbf{(E)}\ 19$

Problem 17

How many ordered pairs $(a, b)$ satisfy

$a\cdot b + lcm(a, b) = 12\cdot gcd(a, b) - a\cdot b$

where $gcd$ denotes the greatest common divisor and $lcm$ denotes the least common multiple?

Problem 18

$\bigtriangleup ABC$ lays flat on the ground and has side lengths $\overline{AB} = 8, \overline{BC} = 15$ , and $\overline{AC} = 17$ . Vertex $A$ is then lifted up creating an elevation angle with the triangle and the ground of $60^{\circ}$ . A wooden pole is dropped from $A$ perpendicular to the ground, making an altitude of a $3$ Dimensional figure. Ropes are connected from the foot of the pole, $D$ , to form $2$ other segments, $\overline{BD}$ and $\overline{CD}$ . What is the volume of $ABCD$ ?