72 + 20 = 70 + 2 + 20 = (70 + 20) + 2 = 90 + 2 = 92

    68 – 40 = 60 + 8 – 40 = (60 – 40) + 8 = 20 + 8 = 28

    350 + 30 = 300 + 50 + 30 = 300 + (50 + 30) = 300 + 80 = 380

    970 – 70 = 900 + 70 – 70 = 900 + (70 – 70) = 900 + 0 = 900