1991+199.1+19.91+1.991
=1991×（1+0.1+0.01+0.001）
=1991×1.111
=（2000-9）×1.111
=2000×1.111-9×1.111
=2222-9.999
=2222-10+0.001
=2212+0.001
=2212.001
或
1991+199.1+19.91+1.991
=(2000-9)+(200-0.9)+(20-0.09)+(2-0.009)
=2222-9.999
=2222-(10-0.001)
=2222-10+0.001
=2212.001