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