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# Elementary Brain Teaser

## 12 Sevens - November 11, 2013

What will be the digit in the tens place of the sum of the following expression: 7 + 77 + 777 + 7777 + ... + 777777777777

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# Middle School Madness

## Red Corner Checker - November 11, 2013

You are given a 5 x 5 checkerboard with red and black squares. If each corner square is red, what percent of the board is black?

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# Algebra in Action

## 100th Group - November 11, 2013

Group the consecutive counting numbers as follows: (1), (2, 3), (4, 5, 6), (7, 8, 9, 10), . . . Note there is one number in the first group, two in the second group, three in the third, etc. Determine the sum of the 100 numbers in the 100th group?

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# Problem of the Week

## 10 Digit Call - November 11, 2013

A typical telephone number in the United States used to be 7 digits. While you can still use a seven-digit code in most areas, a multitude of regions have begun using ten-digit telephone numbers. If the entire country goes to a ten-digit telephone, exactly how many different ten-digit telephone numbers will be available such that the first digit cannot be a 0 or 1 and the fourth digit cannot be a 0 or a 1?