594+187 =(600-6)+(200-13)=(600+200)-(6+13)=800-19=800-20+1=781
398+487 =(400-2)+(500-13)=(400+500)-(2+13)=900-15=900-10-5=885
294+491 = (300-6)+(500-9)=(300+500)-(6+9)=800-15=800-10-5=785
494+387 =(500-6)+(400-13)=(400+500)-(6+13)=900-19=900-20+1=881
487+388=(500-13)+(400-24)=(500+400)-(13+24)=900-37=900-30-7=863