问一道GWD 第7套的第14道数学题

nancyan1985

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

我用排列做的最后得出来的是A 但是答案是E 麻烦大家帮忙看看啊  觉得自己28号要裸考 所以还是得增加实力

biglin1987

the number of 5-letter code words 是A10取5，等于10*9*8*7*6

 the number of 4-letter code words是A10取4，等于10*9*8*7

所以是6：1

nancyan1985

谢谢啊 明白了