(640-240):8+28:7=400:8+28:7=50+4=54
640-(240:8)+28:7=640-30+4=614
640-240:(8+28:7)=640-240:12=640-20=620
(640-240):8+28:7=400:8+28:7=50+4=54
640-(240:8)+28:7=640-30+4=614
640-240:(8+28:7)=640-240:12=640-20=620