短文改错。

     Last summer I learnt to ride a bicycle. At first,I was unable to control it's direction. Sometimes

I fall to the right or to the left. But I asked my father to tell me how I could ride straightly. He didn't

answer me directly. Instead, he took me to side of the street. "Can you find some bike track that is

really straight?" asked my father. I watched more carefully and shook my head ."No" he said, "in fact

neither of the tracks is always straight. But just by correct the direction every now and then, you had

already gone forward."

某企业采用标准成本法，A产品的正常生产能量为1000件，单位产品标准成本如下：

直接材料=0.1×150=15元

直接人工=5×4=20元

制造费用：

单位产品标准成本=15+20+6+5=46元

本月生产A产品800件，实际单位成本为：

直接材料=0.11×140=15.4元

直接人工=5.5×3.9=21.45元

制造费用：

单位产品实际成本=15.4+21.45+5+6.25=48.10元

要求：

计算直接材料成本差异。