$\"\"$

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The annual sales of an electronic games manufacturer is $\"\"$ .

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Each year $\"\"$ of the units that have been sold will become inoperative.

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After two years the number of units are $\"\"$ .

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$\"\"$ .

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After three years the number of units are $\"\"$ .

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$\"\"$ .

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After $\"\"$ years the number of units are $\"\"$ .

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The sum of the first $\"\"$ terms of the geometric series is $\"\"$ .

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$\"\"$ and $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$ for $\"\"$ .

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$\"\"$

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The number of units after $\"\"$ years are $\"\"$ for $\"\"$ .