20. 如图,一个三棱锥S−ABCS-ABCS−ABC中,DDD,EEE,FFF分别为棱SASASA,SBSBSB,SCSCSC上的点,且SD:DA=SE:EB=CF:FS=2:1SD:DA=SE:EB=CF:FS=2:1SD:DA=SE:EB=CF:FS=2:1,则三棱锥S−DEFS-DEFS−DEF的体积与三棱锥S−ABCS-ABCS−ABC的体积之比((( )))