✔ 最佳答案
(a)1C1 * 6C3 = 120
1C1 :
Since the thousand digit must be zero, therefore you can only select one (ie. 0) from the seven number.
6C3 :
As 0 is used to occupy thousand digit, you can select three number from the remaining six number. (ie. 1~6)
------------------------------------------------------------------------------------
(b)6C1 * 6C3 = 720
6C1 :
Since the thousand digit can be anything except zero, therefore you can select one number among 1~6.
6C3 :
You can select three number from the remaining six numbers for the other digits.
2012-05-17 14:19:26 補充:
Forgot to tell C means combination.
For example: 6C3 = 6!/3! = 6*5*4*3*2*1/3*2*1 = 6*5*4 = 120
2012-05-17 14:28:07 補充:
For this question, you can check whether you calculated right or wrong.
Since there are 7 numbers to form 3-digit or 4-digit number,
(It cannot be less than 3-digit as there is only one "0")
therefore the total combination = 7C4 = 7!/(7-4)! = 7!/3! (=7*6*5) = 840, which is the sum of (a) & (b)
2012-05-17 14:30:28 補充:
sorry 7!/3! = 7*6*5*4, not 7*6*5