6.9+0.1-6.9+0.1=6.9-6.9+0.1+0.1=0.2
=6.9-6.9+0.1+01.=0+0.2=0.2
=6.9-6.9+0.1+0.1=0.2
6.9+6.9一0.1+0.1=20.1
=6·9-6·9+0.1+0.1=0.2