2.400x30= 450x600= 80x926= 200x75= 190x300+929x40= 720x100-28.142:2=

400 x 30 = 12 000;450 x 600 = 400 * 600 + 50 * 600 = 240 000 + 30 000 = 270 000;80 x 926 = 80 * 900 + 80 * 26 = 72 000 + 80 * 26 = 72 000 + 80 * 20 + 80 * 6 = 72 000 + 1600 + 480 = 73 600 + 480 = 74 000 + 80 = 74 080;200 x 75 = 200 * 70 + 200 * 5 = 14000 + 1000 = 15 000;190 x 300 + 929 x 40 = 100 * 300 + 90 * 300 + 900 * 40 + 29 * 40 = 30000 + 27 000 + 36 000 + 20 * 40 + 9 * 40 = 57 000 + 36 000 + 800 + 360 = 93 000 + 800 + 200 + 160 = 94 160; 720 x 100 - 28 = 700 * 100 + 20 * 100 - 28 = 70000 + 2000 - 28 = 71 900 + 100 - 28 = 71 900 + 72 = 71 972;142 : 2 = 100 / 2 + 42 / 2= 50 + 40 / 2 + 2 / 2 = 50 + 20 + 1 = 71.