21*(1+90%）/8分之3-106.4
=21*1.9/(3/8)-106.4
=21*19/10/(3/8)-106.4
=(20+1)(20-1)/10/(3/8)-106.4
=(400-1)/10/(3/8)-106.4
=399/10/(3/8)-106.4
=39.9/(3/8)-106.4
=39.9*(8/3)-106.4
=13.3*8-106.4
=106.4-106.4
=0