sql - Help! MySQL Query -

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i need single line query. must dumb or over-thinking mysql query. please solving following query.

lets there 2 simple tables: class , books

class
id--name-----students
1------kg-----------20
2------grade(3)---25
3------grade(5)---30

books
id--classid--title-------------cost
1-----1--------------drawing------------------5
2-----3--------------history--------------------25
3-----1--------------a-to-z--------------------10
4-----2--------------alphabets---------------20
5-----3--------------maths--------------------15
6-----2--------------english-------------------30

lets say:
know -----> id of class
have find ---> book cost of class. (books each students in class)

can have single line of query?

try this:

in single line:

select sum(class.students * books.cost) bookcost  books inner join class       on books.classid = class.classid books.classid = <class-id-value>  group books.classid

with formatting: 

select sum(class.students * books.cost) bookcost   books inner join class    on books.classid = class.classid books.classid = <class-id-value> group books.classid

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