1880*199.9-1999*187.9
=188*1999-1999*187.9
=（188-187.9）*1999
=0.1*1999
=199.9
6.42*1.01/6.42
=6.42/6.42*1.01
=1*1.01
=1.01
12.5*8.8
=12.5*8*1.1
=100*1.1
=110