## Problem of the Day #378: Filling a Grid, Part IMarch 31, 2012

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Given a grid with $R$ rows and $C$ columns, what is the maximum number of cells that can be colored such that no colored cell is adjacent to more than two other colored cells? Express your answer in terms of $R$ and $C$. Two cells are adjacent if they share an edge.

## Problem of the Day #377: Happy $n_b^\text{th}$ Birthday!March 30, 2012

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Sreenath has foolishly forgotten how old Jon is. He knows Jon is $57$, $73$, or $91$, but can’t remember which. In an attempt to figure it out, Sreenath poses a clever math problem for Jon on his birthday (today). “Happy $n_b^{\text{th}}$ Birthday, Jon!” Sreenath says (where $b$, the base of $n$, is an integer greater than 2). Solve for $n$, given that it is three digits long.

## Problem of the Day #376: Log Entry 01March 29, 2012

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Alex has decided to provide you with the following data:

$$\begin{eqnarray} \ln(2) & \approx & 0.693147181 \\ \ln(3) & \approx & 1.09861229 \\ \ln(5) & \approx & 1.60943791 \\ \ln(7) & \approx & 1.94591015 \end{eqnarray}$$

Your mission, should you choose to accept it, is to find the number of digits in the decimal expansion of $35^{12000}$.

## Problem of the Day #375: Log Entry 00March 28, 2012

Posted by Albert in : potd , add a comment

Alex has decided to provide you with the following data:

$$\begin{eqnarray} \ln(2) & \approx & 0.693147181 \\ \ln(3) & \approx & 1.09861229 \\ \ln(5) & \approx & 1.60943791 \\ \ln(7) & \approx & 1.94591015 \end{eqnarray}$$

Your mission, should you choose to accept it, is to find the smallest positive integer $x$ such that $2^{10 x}$ has more than $3x+1$ digits.

## Problem of the Day #374: Increasing Sequence NumbersMarch 27, 2012

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Find the number of integers satisfying the property that the concatenation of all the odd-indexed digits (the leftmost digit is index 1), starting from the leftmost digit, followed by all the even-indexed digits, starting from the rightmost even-indexed digit, forms a strictly increasing sequence.

## Problem of the Day #373: Palindromey StringsMarch 26, 2012

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A palindromey string is a string of lowercase letters that is either a palindrome or the concatenation of two palindromey strings. Find the number of palindromey strings of length $12$.

## Problem of the Day #372: Binary Tree Path LengthsMarch 25, 2012

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Inspired by a problem from the 2012 TJ IOI.

The nodes in a complete binary tree of infinite height are referred to by ordered pairs $(x, y)$, where $x$ and $y$ are 0-indexed values denoting the row and the column of the nodes. The root is $(0, 0)$. Find the length of the shortest path between node $(1000, 3)$ and $(1234, 123456)$.

## Problem of the Day #371: Connecting ComponentsMarch 24, 2012

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Suppose we have a list of $100$ nodes. Every second, two nodes are randomly chosen and an edge is drawn in between the two nodes. What is the expected number of seconds that must elapse before the number of connected components of the graph is less than or equal to $5$?

## Problem of the Day #370: Rows of CountersMarch 23, 2012

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Alex is playing a game in which he manipulates a row of black and white counters. To change the row, he is allowed to remove any black counter and replace it with either a white counter and a black counter (in that order), or two black counters. He is not allowed to perform any other manipulations.

If he starts with a single black counter, how many distinct rows of $10$ counters can he arrange?

## Problem of the Day #369: Circumradius of a PyramidMarch 22, 2012

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If a right pyramid with a square base and angles of $120^{\circ}$ between its faces has an inradius of length $1$, find its circumradius.