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Puzzles

21 December

There are 6 two-digit numbers whose digits are all 1, 2, or 3 and whose second digit onwards are all less than or equal to the previous digit:
How many 20-digit numbers are there whose digits are all 1, 2, or 3 and whose second digit onwards are all less than or equal to the previous digit?

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15 December

The arithmetic mean of a set of \(n\) numbers is computed by adding up all the numbers, then dividing the result by \(n\). The geometric mean of a set of \(n\) numbers is computed by multiplying all the numbers together, then taking the \(n\)th root of the result.
The arithmetic mean of the digits of the number 132 is \(\tfrac13(1+3+2)=2\). The geometric mean of the digits of the number 139 is \(\sqrt[3]{1\times3\times9}\)=3.
What is the smallest three-digit number whose first digit is 4 and for which the arithmetic and geometric means of its digits are both non-zero integers?

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10 December

How many integers are there between 100 and 1000 whose digits add up to an even number?

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3 December

190 is the smallest multiple of 10 whose digits add up to 10.
What is the smallest multiple of 15 whose digits add up to 15?

23 December

How many numbers are there between 100 and 1000 that contain no 0, 1, 2, 3, or 4?

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11 December

There are five 3-digit numbers whose digits are all either 1 or 2 and who do not contain two 2s in a row: 111, 112, 121, 211, and 212.
How many 14-digit numbers are there whose digits are all either 1 or 2 and who do not contain two 2s in a row?

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6 December

There are 21 three-digit integers whose digits are all non-zero and whose digits add up to 8.
How many positive integers are there whose digits are all non-zero and whose digits add up to 8?

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3 December

Write the numbers 1 to 81 in a grid like this:
$$ \begin{array}{cccc} 1&2&3&\cdots&9\\ 10&11&12&\cdots&18\\ 19&20&21&\cdots&27\\ \vdots&\vdots&\vdots&\ddots&\vdots\\ 73&74&75&\cdots&81 \end{array} $$
Pick 9 numbers so that you have exactly one number in each row and one number in each column, and find their sum. What is the largest value you can get?

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