Question: Find the average age of the students who have allergies with food and animal types.
SQL: SELECT avg(age) FROM Student WHERE StuID IN ( SELECT T1.StuID FROM Has_allergy AS T1 JOIN Allergy_Type AS T2 ON T1.Allergy  =  T2.Allergy WHERE T2.allergytype  =  "food" INTERSECT SELECT T1.StuID FROM Has_allergy AS T1 JOIN Allergy_Type AS T2 ON T1.Allergy  =  T2.Allergy WHERE T2.allergytype  =  "animal")