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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A signal can be made using 3 flags. We have total of 4 , so 3 can be chosen out of 4 in 4C3 = 4C1 = 4 ways The three flags can be arranged in 3 ! ways to make a unique combination. Hence 4 * 3! = 24 ways.
因为有特定的顺序，所以这是排列，从4面旗子中选三个排列，就是A43=24
assume the signal is ABC so A has 4 cases B has 3 cases C has 2 cases so there are 4x3x2=24
4面可选择的旗子，从中选3面，顺序有影响，所以是4*3*2
a43, 全排列
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A signal can be made using 3 flags. We have total of 4 , so 3 can be chosen out of 4 in 4C3 = 4C1 = 4 ways
The three flags can be arranged in 3 ! ways to make a unique combination. Hence 4 * 3! = 24 ways.
因为有特定的顺序，所以这是排列，从4面旗子中选三个排列，就是A43=24
assume the signal is ABC
so A has 4 cases
B has 3 cases
C has 2 cases
so there are 4x3x2=24
4面可选择的旗子，从中选3面，顺序有影响，所以是4*3*2
a43, 全排列