Решение:
а)
$$72:(12+6)\cdot 4 = 72:18 \cdot 4 = 4 \cdot 4 = 16$$
$$72:(12 + 6\cdot4) = 72 : (12 + 24) = 72 : 36 = 2$$
$$(72:12 + 6) \cdot 4 = (6 + 6) \cdot 4 = 12 \cdot 4 = 48$$
$$72:12 + (6 \cdot 4) = 6 + 24 = 30$$
б)
$$(120-40):5\cdot 2 = 80:5 \cdot 2 = 16 \cdot 2 = 32$$
$$120 - (40 : 5 \cdot 2) = 120 - (8 \cdot 2) = 120 - 16 = 104$$
$$120 - 40 : (5 \cdot 2) = 120 - 40 : 10 = 120 - 4 = 116$$
$$(120 - 40:5)\cdot 2 = (120 - 8) \cdot 2 = 112 \cdot 2 = 224$$