5*31+25*31 , 19*35+1*35 , 17*14+13*14 , 17*23-7*23 , 315*6-15*6 , 4*129-3*129 ,47*6-7*6 , 9*21-9*11 , 5*24+5*16 столбиком снизу и легким

5 * 31 + 25 * 31 = 31 * (5 + 25) = 31 * 30 = 930
19 * 35 + 1 * 35 = 35 * (19 + 1) = 35 * 20 = 700
17 * 14 + 13 * 14 = 14 * (17 + 13) = 14 * 30 = 420
17 * 23 - 7 * 23 = 23 * (17 - 7) = 23 * 10 = 230 
315 * 6 - 15 * 6 = 6 * (315 - 15) = 6 * 300 = 1800
4 * 129 - 3 * 129 = 129 * (4 - 3) = 129 * 1 = 129
47 * 6 - 7 * 6 = 6 * (47 - 7) = 6 * 40 = 240
9 * 21 - 9 * 11 = 9 * (21 - 11) = 9 * 10 = 90
5 * 24 + 5 * 16 = 5 * (24 + 16) = 5 * 40 = 200
Столбик здесь не нужен.