MySQL Query for book cost of a class

Question

I just need a single MySQL line query for the following.

Lets say there are 2 simple tables: Class and Books<

Answer 1

   SELECT c.Name,
          SUM(b.Price * c.Students) cost
     FROM Class c
LEFT JOIN Books b ON b.ClassId = c.ID
    WHERE c.Students >= 31
 GROUP BY c.ID