6_6_4=9; 6_6_4=5; 8_6_7=2; 6_6_4=3

6*6/4=9; 6/6+4=5; 8+6/7=2; 6+6/4=3

6+6-4=9, 6:6+4=5, 8+6:7=2, 6+6:4=3.