5分之2除（负2又5分之2）减21分之8乘（负1又4分之3）减0.5除2乘2分之1
=-2/5x5/12+8/21x7/4-1/2x2/5
=-1/6+2/3-1/5
=-5/30+20/30-6/30
=9/30
=3/10