EX.12->
a)317+ =914-220
914-220=694
694-317=377 => 317+377=914-220
520- =870-354
870-354=516
520-516=4 => 520-4=870-354
189+ =752+164
752+164=916
916-189=727 => 189+727=752+164
b) +3.055=19.870-14.200
19.870-14.200=5.670
5.670-3.055=2.615 => 2.615+3.055=19.870-14.200
-1.023=10.125+12.360
10.125+12.360=22.485
22.485+1.023=23.508 => 23.508-1.023=10.125+12.360
+5.790=41.839-24.517
41.839-24.517=17.322
17.322-5.790=11.532 => 11.532+5.790=41.839-24.517