Value to be from a selection of 3 and not null : I'm using oracle to create a table called module:

Module(ModuleId, ModuleTitle, ModuleLeader, Credits, CourseworkPercentage, ExamPercentage)

ModuleId being the PK.

A constraint/check which i have to implement is for 'Credits' to be not null and it must be equal to one of the following three values: 10, 20 or 40. 
What I have so far is:



    CREATE TABLE  "MODULE_CC" 
    ("MODULEID" NUMBER, 
	"MODULETITLE" VARCHAR2(30) NOT NULL ENABLE, 
	"MODULELEADER" VARCHAR2(30) NOT NULL ENABLE, 
	"CREDITS" NUMBER NOT NULL ENABLE, 
	"COURSEWORKPERCENTAGE" NUMBER, 
	"EXAMPERCENTAGE" NUMBER, 
	 PRIMARY KEY ("MODULEID") ENABLE, 
	 CHECK (CourseworkPercentage + ExamPercentage = 100) ENABLE
     );


What would be the check/constrains to make sure that credits will be equal to one of three values(10,20 or 40)?