maths and stat

用戶40491

2010-06-16 5:51 am

1) If 4 letters and selected from the 7 letters of the word"WINDOWS".calculate the number of possible

a)combinations, (ans:25)
b)arrangements (ans:480)

2)A disk box contains 10 floppy disks of which 3 are black and the other 7 are each of different colours. 8 disks are selected from the box. What is the number of possible different selections?? (ans:29)

Please teach with with explanation.Thanks a lot !!!

回答 (1)

wing hang

2010-06-16 5:44 pm

✔ 最佳答案

1) If 4 letters and selected from the 7 letters of the word"WINDOWS".calculate the number of possible

a)combinations, (ans:25)
There has 2W1I1N1D1O1S
combinations
case1:1個W
其他3個LETTERS就係餘下5個抽3個
所以有5C3=10
case2:2個W抽2個W
其他2個LETTERS就係餘下5個抽2個
所以有5C2=10
case3:冇W被抽
所以有5C4=5
所以共有=10+10+5=25組合

b)arrangements (ans:480)
case1: 1個w
可能性有=5x4x3x(4C1)=240
case2: 2個w
可能性有=(5x4)(4C2)=120
case3: 0個w
可能性有=5P4=5x4x3x2=120

共有=240+120+120=480

2)A disk box contains 10 floppy disks of which 3 are black and the
other 7 are each of different colours. 8 disks are selected from the
box. What is the number of possible different selections?? (ans:29)
分3個情況計算:

1) 0黑 - 7C7=1

2) 1黑 - 7C6=7

3) 2黑 - 7C5=21

3個情況加埋=1+7+21=29

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