请教数学题

zhangyer

If a code word is defined to be a sequence of different letters chosen from the 10 letters A, B, C, D, E, F, G, H, I, and J, what is the ratio of the number of 5-letter code words to the number of 4-letter code words?

 

A.     5 to 4

B.     3 to 2

C.     2 to 1

D.     5 to 1

E.      6 to 1

key is E.

Why?

hebeqiao

P^510/p^4 10=6

usababy

6：1
10个里选5个的排列是 10×9×8×7×6
10个里选4个的排列是10×9×8×7
比值是6：1

zhangyer

多谢！！！