15 O 9 O 7 = 13
20 O 3 O 6 = 17
54 O 5 O 4 = 45
42 O 6 O 6 = 30
37 O 8 O 5 = 40
28 O 7 O 6 = 29
15 − 9 + 7 = 6 + 7 = 13
20 + 3 − 6 = 23 − 6 = 17
54 − 5 − 4 = 50 − 5 = 45
42 − 6 − 6 = 42 − 12 = 30
37 + 8 − 5 = 45 − 5 = 40
28 + 7 − 6 = 35 − 6 = 29