50%乘（20—（7分之2—4分之3除45%））
=0.5*(20-2/7+3/4*20/9)
=0.5*(20-2/7+5/3)
=10-1/7+5/6
=9又42分之13
20%乘0.37+63%乘5分之1+4%
=0.2*0.37+0.63*0.2+0.04
=0.2*（0.37+0.63）+0.04
=0.2*1+0.04
=0.24