Each signal that a certain ship can make is comprised of 3 different flags hanging vertically in a particular order. How many unique signals can be made by using 4 different flags?
10
12
20
24
36
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4面可选择的旗子，从中选3面，顺序有影响，所以是4*3*2
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4面可选择的旗子，从中选3面，顺序有影响，所以是4*3*2
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