实验过程：

Ⅰ．打开弹簧夹，打开活塞a，滴加浓盐酸．

Ⅱ．当B和C中的溶液都变为黄色时，夹紧弹簧夹．

Ⅲ．当B中溶液由黄色变为棕红色时，关闭活塞a．

Ⅳ．…

20－ $\frac{\text{5}}{\text{4}}$ × $\frac{\text{12}}{\text{5}}$ =____

    Nobody in the street knew Miss Hilton. While she lived, her front gate was always locked and no one ever saw her 1 or saw 2 go in. So even if you 3, you couldn't feel sorry and 4 that you 5 Miss Hilton.

    When I think of 6, I see just two colours, grey and green. The green of mango tree, the grey of the house and the grey of the high iron fence（铁栅栏） that kept you off the mangoes.

    If your football 7 Miss Hilton's garden, you never 8. It wasn't the mango season 9 Miss Hilton died. But we got back about ten 10 twelve of our footballs.

    11 the end of the week a sign was shown on the mango tree: For sale.

    We were ready to dislike 12 even before they came. I think we 13. Already we had one man who kept on complaining（抱怨）about us 14 the police. He complained that we played football near his house and if we weren't playing football he complained that we were making 15 noise anyway.