a) 993+(455+148)=993+603 = 1596
187+(870-230)= 187+640 = 827
7500-(1250+1250) = 7500-2500 = 5000
b) 2845-829+2400 = 4416
15780-(1075+2750) = 15780-3825 = 11955
6204+(3000-2200) = 6204+800 = 7004
c) 7911-[8211-(10100-5702)] = 7911-(8211-4398) = 7911-3813 = 4098
12563-[1456+(9876-867)] = 12563-(1456+9009) = 12563-10465 = 2098
7015-[3015-(2004-4)] = 7015-(3015-2000) = 7015-1015 = 6000
