如图
图
see
(1/2-1/5+1/4)x60=1/2x60-1/5x60+1/4x60=30-12+15=33
(1/2-1/5+1/4)×60=(1/2)x60 -(1/5)x60 + (1/4)x60=30-12+15=30+3=33