18)
а.21х14+21х6=21х(14+6)=420
б.77х1298-77х1198=77х(1298-1198)=7700
в.-90х88-90х12=-90х(88+12)=-9000
г.-777х330-670х777=-777х(330+670)
17)
а.-9х(-15+(-45))=540
б.-18х(80+20)=1800
в.87х(-72+(-28))=8700
г.110х(-6+(-2)+(-2))=1100
19)
а.43х(32-22)-53х(52+42)=4552
б.68х(2-15-5)+24=-1200
в.45х(-12-88)+75(88+12)=3000