Discrete Mathematics
# Rule of Sum and Rule of Product

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When we write the integers from 1 to 100, how many times does the digit 9 appear?

$n \times n$ square with $n \geq 2$ ?

Which of the following expresses the correct formula for the number of unit squares with exactly 1 red side in anFind the number of $2$ digit numbers whose digit sum is a perfect square.

$3 \times 3 \times 3$ Rubik cube and break it up into the $3^3$ cubes, how many cubes would have stickers on exactly $2$ faces?

If you take a**Bonus question**: Can you generalize this for a $N \times N \times N$ Rubik cube?