The average age of all the teachers in Gotia School is $34$. There are $3$ male teachers in Gotia School with an average age of $27$. The average age of female teachers is $35$. How many female teachers are there? 
Overseas Competition->Knowledge Point->Word Problem Modules->Average Problems 
Total age less than the average: $(34-27)\times3=21$.  Thus, there are $21\div(35-34)=21$ female teachers. 