question: Find the average age and number of male students (with sex M) from each city.
#1 return cities 
#2 return students from  #1 
#3 return sex of  #2 
#4 return #2 where #3 is  M 
#5 return ages of  #4 
#6 return average of  #5 for each  #1 
#7 return number of  #4 for each  #1 
#8 return #6 ,  #7