Cindy has five $5coins,three $2 coins and two $1 coins in her purse.
问题描述：
Cindy has five $5coins,three $2 coins and two $1 coins in her purse.
（a）How many$ 2 coins should be added to her purse so that their expected value of a coin randomly chosen from her purse is $3.
答
答案是x先翻译一下吧.Cindy有五个5元币,三个2元币和两个1元币.
问题是放多少个2元币进去能使expected value 即平均值 为3元?
先设个数为X
那么总金额为 5*5+3*2+2*1+2*x=33+2x
金币总数为5+3+2+x=10+x
由问题得
(33+2x)/(10+x)=3
解得x=3为什么不是 5*5+3*2+2*1+2*（x+3）=（10+x）*3 呢？因为他原本就有三个2元币啊！？