question: Find the average age of the students who have allergies with food and animal types.
#1 return students 
#2 return #1 who  have  allergies 
#3 return #2 with  food 
#4 return #2 with  animals 
#5 return #3 ,   #4 
#6 return the ages of  #5 
#7 return the average of  #6