199.4×19.93-199.3×19.92=(199.3+0.1)×19.93-199.3×(19.93-0.01)=199.3×19.93+19.93×0.1-199.3×19.93+199.3×0.01=1.993+1.993=(2-0.007)+(2-0.007)=4-0.014=3.986