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numbers and algebraic expressions
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trigonometry
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combinatorics
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elements of statistics
kropki game

Divide some action into two parts. If first part we can do in \wz{$n$} ways and second in k ways then whole action we can do in n⋅k. ways. Example. How many two digit number can be formed? 1st part: Choose first digit in 9 ways (whithout zero). 2nd etap: Choose second digit in 10 ways. There are 90 two digit numbers. 9⋅10 = 90. Divide some action into three parts. If first part we can do in n ways, second in k ways and third in k ways then whole action we can do in n⋅k⋅p ways. There is similar when some action we can do in four, five,... parts.