398*34 - 49*68 =
398*34=400*35-400*(35-34)-(400-398)*34=14000-400*1-2*34=14000-400-68=13600-(100-32)= 13600-100+32=13532
49*68=50*70-50*(70-68)-(50-49)*68=3500-50*2-1*68= 3500-100-68= 3400-(100-32)=3400-100+32=3332
13532-3332=10000+3532-3332=10000+200=10200