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.
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