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If the answer is 12 inches long the answer would be 3. She would have 3 sandwiches.

12 inches = 1 foot


3 assuming that you meant 12 inches long instead of each piece being 12 feet long. 
You would need to know that 1 foot= 12inches which means that every piece is really a foot long. 
So, 3 foot long sandwich÷ 1 foot per piece= 3 pieces

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