68 × 98= 68 × (100 - 2)= 68 × 100 - 68 × 2= 6800 - (70 - 2)× 2= 6800 - 70 × 2 + 2 × 2= 6800 - 140 + 4= 6660 + 4= 6664
68(100-2)=6800-136=6664
98写成100-2