$\"\"$

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The expression is $\"\"$ .

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The number of combinations formula: $\"\"$ .

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$\"\"$

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$\"\"$ $\"\"$

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The fundamental counting principle $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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$\"\"$

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= 105

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$\"\"$

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The solution is 105.