我是用手机写的,有的地方也写不明白,凑合看吧,1997*0.3/1999*0.5 1.2*0.5/1999*0.5=1997*0.3/1999*0.5 0.3*4*0.5/1999*0.5=1997*0.3/1999*0.5 2*0.3/1999*0.5=(1997＋2)*0.3/1999*0.5=3/5