回复：数学题求救，先谢了

The first question to answer is: how many 7 digit numbers are possible in base 10.

if zero cannot be used in the first position and all digits can repeat in the number, then there are 9 * 10 * 10 * 10 * 10 * 10 * 10 possible numbers.

if zero can be used in the first position and all digits can repeat in the number, then there are 10 * 10 * 10 * 10 * 10 * 10 * 10 possible numbers.

if zero cannot be used in the first position and no digits can repeat in the number, then there are 9 * 9 * 8 * 7 * 6 * 5 * 4 possible numbers.

if zero can be used in the first position and no digits can repeat in the number, then there are 10 * 9 * 8 * 7 * 6 * 5 * 4 possible numbers.

for the question being asked, where 1 digit can repeat exactly 4 times and zero can be used in the first position, then there would be 10 * 10 * 10 * 10 * 9 * 8 * 7 possible numbers.

when 2 digits can repeat exactly twice and zero can be used in the first position, then there would be 10 * 10 * 9 * 9 * 8 * 7 * 6 possible numbers.

 

所有跟帖： 

• F -Kamioka- ♀ (0 bytes) () 04/12/2012 postreply 09:55:07

• 希望这个给你一个解答了。。。 -SINEAD4273- ♀ (0 bytes) () 04/12/2012 postreply 09:55:32

• 你的大学数学老师这时候一定在偷偷抹眼泪，哈哈～～ -Kamioka- ♀ (0 bytes) () 04/12/2012 postreply 09:57:33

• 请仔细读题。 -柯西- ♀ (0 bytes) () 04/12/2012 postreply 10:14:18