What are the number of games played for students under the age of 20?
SELECT sum ( gamesplayed )  FROM Sportsinfo AS T1 JOIN Student AS T2 ON T1.StuID   =   T2.StuID WHERE T2.age  <  20
number of games played for students under the age of 20 refers to SUM(gamesplayed); students under the age of 20 refers to age < 20; joining tables Sportsinfo and Student on StuID.