10x(12+15)=
9x(21-13)=
5x(5+3)=
7x(100-10)=
3x(1+5)=
4x(16-10)=
4x16-4x10=64-40=24
9x21-9x13=189-117=72
5x5+5x3=25+15=40
7x100-7x10=700-70=630
10x12+10x15=120+150=270
3x1+3x5=3+15=18
