12x1/5=2.4(12-2.4)x1/4=2.4两次一共用了4.8吨
12*(1/5+1/4)=12*9/20=5.4吨两次一共用去5.4吨
一共=12×(1/5+1/4)=4.5吨
4.8吨
5.4吨
12*1/5+12*1/4=5.4吨