19.99*19.98-19.97*19.96
=(19.97+0.02)*19.98-19.97*(19.98-0.02)
=(19.97*19.98+0.02*19.98)-(19.97*19.98-19.97*0.02)
=19.97*19.98+0.02*19.98-19.97*19.98+19.97*0.02
=0.02*19.98+19.97*0.02
=0.02*(19.98+19.97)
=0.02*(20-0.02+20-0.03)
=0.02*(40-0.05)
=0.02*40-0.02*0.05
=0.8-0.01
=0.799