# Difference between revisions of "2012 AMC 12A Problems"

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== Problem 9 == | == Problem 9 == | ||

+ | |||

+ | A year is a leap year if and only if the year number is divisible by <math>400</math> (such as <math>2000</math>) or is divisible by <math>4</math> but not by <math>100</math> (such as <math>2012</math>). The <math>200\text{th}</math> anniversary of the birth of novelist Charles Dickens was celebrated on February <math>7</math>, <math>2012</math>, a Tuesday. On what day of the week was Dickens born? | ||

+ | |||

+ | <math> \textbf{(A)}\ \text{Friday} | ||

+ | \qquad\textbf{(B)}\ \text{Saturday} | ||

+ | \qquad\textbf{(C)}\ \text{Sunday} | ||

+ | \qquad\textbf{(D)}\ \text{Monday} | ||

+ | \qquad\textbf{(E)}\ \text{Tuesday} | ||

+ | </math> | ||

== Problem 10 == | == Problem 10 == | ||

+ | |||

+ | <math> \textbf{(A)}\ \frac{3}{10}\qquad\textbf{(B)}\ \frac{1}{3}\qquad\textbf{(C)}\ \frac{9}{20}\qquad\textbf{(D)}\ \frac{2}{3}\qquad\textbf{(E)}\ \frac{9}{10} </math> | ||

== Problem 11 == | == Problem 11 == | ||

+ | |||

+ | Alex, Mel, and Chelsea play a game that has <math>6</math> rounds. In each round there is a single winner, and the outcomes of the rounds are independent. For each round the probability that Alex wins is <math>\frac{1}{2}</math>, and Mel is twice as likely to win as Chelsea. What is the probability that Alex wins three rounds, Mel wins two rounds, and Chelsea wins one round? | ||

+ | |||

+ | <math> \textbf{(A)}\ \frac{5}{72}\qquad\textbf{(B)}\ \frac{5}{36}\qquad\textbf{(C)}\ \frac{1}{6}\qquad\textbf{(D)}\ \frac{1}{3}\qquad\textbf{(E)}\ 1 </math>[/ | ||

== Problem 12 == | == Problem 12 == | ||

+ | |||

+ | A square region <math>ABCD</math> is externally tangent to the circle with equation <math>x^2+y^2=1</math> at the point <math>(0,1)</math> on the side <math>CD</math>. Vertices <math>A</math> and <math>B</math> are on the circle with equation <math>x^2+y^2=4</math>. What is the side length of this square? | ||

+ | |||

+ | <math> \textbf{(A)}\ \frac{\sqrt{10}+5}{10}\qquad\textbf{(B)}\ \frac{2\sqrt{5}}{5}\qquad\textbf{(C)}\ \frac{2\sqrt{2}}{3}\qquad\textbf{(D)}\ \frac{2\sqrt{19}-4}{5}\qquad\textbf{(E)}\ \frac{9-\sqrt{17}}{5} </math> | ||

== Problem 13 == | == Problem 13 == |

## Revision as of 15:50, 11 February 2012

## Contents

- 1 Problem 1
- 2 Problem 2
- 3 Problem 3
- 4 Problem 4
- 5 Problem 5
- 6 Problem 6
- 7 Problem 7
- 8 Problem 8
- 9 Problem 9
- 10 Problem 10
- 11 Problem 11
- 12 Problem 12
- 13 Problem 13
- 14 Problem 14
- 15 Problem 15
- 16 Problem 16
- 17 Problem 17
- 18 Problem 18
- 19 Problem 19
- 20 Problem 20
- 21 Problem 21
- 22 Problem 22
- 23 Problem 23
- 24 Problem 24
- 25 Problem 25

## Problem 1

A bug crawls along a number line, starting at -2. It crawls to -6, then turns around and crawls to 5. How many units does the bug crawl altogether?

## Problem 2

Cagney can frost a cupcake every 20 seconds and Lacey can frost a cupcake every 30 seconds. Working together, how many cupcakes can they frost in 5 minutes?

## Problem 3

A box centimeters high, centimeters wide, and centimeters long can hold grams of clay. A second box with twice the height, three times the width, and the same length as the first box can hold grams of clay. What is ?

## Problem 4

In a bag of marbles, of the marbles are blue and the rest are red. If the number of red marbles is doubled and the number of blue marbles stays the same, what fraction of the marbles will be red?

## Problem 5

A fruit salad consists of blueberries, raspberries, grapes, and cherries. The fruit salad has a total of pieces of fruit. There are twice as many raspberries as blueberries, three times as many grapes as cherries, and four times as many cherries as raspberries. How many cherries are there in the fruit salad?

## Problem 6

The sums of three whole numbers taken in pairs are , , and . What is the middle number?

## Problem 7

Mary divides a circle into sectors. The central angles of these sectors, measured in degrees, are all integers and they form an arithmetic sequence. What is the degree measure of the smallest possible sector angle?

## Problem 8

An *iterative average* of the numbers , , , , and is computed in the following way. Arrange the five numbers in some order. Find the mean of the first two numbers, then find the mean of that with the third number, then the mean of that with the fourth number, and finally the mean of that with the fifth number. What is the difference between the largest and smallest possible values that can be obtained using this procedure?

## Problem 9

A year is a leap year if and only if the year number is divisible by (such as ) or is divisible by but not by (such as ). The anniversary of the birth of novelist Charles Dickens was celebrated on February , , a Tuesday. On what day of the week was Dickens born?

## Problem 10

## Problem 11

Alex, Mel, and Chelsea play a game that has rounds. In each round there is a single winner, and the outcomes of the rounds are independent. For each round the probability that Alex wins is , and Mel is twice as likely to win as Chelsea. What is the probability that Alex wins three rounds, Mel wins two rounds, and Chelsea wins one round?

[/

## Problem 12

A square region is externally tangent to the circle with equation at the point on the side . Vertices and are on the circle with equation . What is the side length of this square?

## Problem 13

## Problem 14

## Problem 15

## Problem 16

## Problem 17

## Problem 18

## Problem 19

## Problem 20

## Problem 21

Let , , and be positive integers with such that and

What is ?

## Problem 22

## Problem 23

Let be the square one of whose diagonals has endpoints and . A point is chosen uniformly at random over all pairs of real numbers and such that and . Let be a translated copy of centered at . What is the probability that the square region determined by contains exactly two points with integer coefficients in its interior?

## Problem 24

Let be the sequence of real numbers defined by , and in general,

\[a_k=\left\{\array{c}(0.\underbrace{20101\cdots 0101}_{k+2\text{ digits}})^{a_{k-1}}\qquad\text{if k is odd,}\\(0.\underbrace{20101\cdots 01011}_{k+2\text{ digits}})^{a_{k-1}}\qquad\text{if k is even.}\] (Error compiling LaTeX. ! Missing $ inserted.)

Rearranging the numbers in the sequence in decreasing order produces a new sequence . What is the sum of all integers , , such that

$\textbf{(A)}\ 671\qquad\textbf{(B)}\ 1006\qquad\textbf{(C)}\ 1341\qquad\textbf{(D)}\ 2011\qquad\textbf{(E)}\2012$ (Error compiling LaTeX. ! Undefined control sequence.)

## Problem 25

Let where denotes the fractional part of . The number is the smallest positive integer such that the equation has at least real solutions. What is ? **Note:** the fractional part of is a real number such that and is an integer.

$\textbf{(A)}\ 30\qquad\textbf{(B)}\ 31\qquad\textbf{(C)}\ 32\qquad\textbf{(D)}\ 62\qquad\textbf{(E)}\64$ (Error compiling LaTeX. ! Undefined control sequence.)