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?
5 to 4
3 to 2
2 to 1
5 to 1
6 to 1
选五个不同字母的情况有10*9*8*7*6种
选四个不同字母的情况有10*9*8*7种
两者相除=6
10p5 / 10p4
10!/(10-5)! : 10!/(10-4)!
差了一个6
∴6 :1