陈向东数学中的概率题

bigwish

陈向东数学中的概率题，请教大家：

有三名打字员为4个科室打字，如果4个科室有一份文件要打，各科室打字员的选择是随机的，问每个打字员都收到文件的概率？

请教思路。

smalllittle

Probability of everyone having received work
= (# of ways for everyone to have some work) / (# of ways to arrange the work)
= (# of ways for 1 person getting 2 files + 2 people each getting 1 file) / (# of ways to distribute the 4 files)