6.7×9.9=6.7×(10-0.1)=67-0.67=66.33
6.7*9.9=6.7*（10-0.1）=6.7*10-6.7*0.1=67-0.67=66.33
【速算】6.7×9.9=67x（10-0.1）=67-0.67=66+0.33=66.33
6.7*(10-0.1)=6.7*10-6.7*0.1=67-0.67=66.33
