一个容器装满100克浓度为80%的溶液，从中取出40克，然后用清水将_爱下问搜题

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问题单项选择题

一个容器装满100克浓度为80%的溶液，从中取出40克，然后用清水将容器倒满，这样反复操作三次，问最后容器中溶液的浓度是多少( )

A．11.52%
B．17.28%
C．28.8%
D．48%

答案

参考答案：B

解析：从题意可知，最后容器中溶液的重量仍为100克，因此，只需求出最后溶液中含有多少溶质，即可求得最后溶液的浓度。而要求剩下的溶质的重量，则需求出三次倒出的溶液中含有多少溶质，每次倒出的溶液虽然都是40克，但是由于浓度不同，其含溶质的量并不完全相同。
原来溶液中含溶质的量为：100×80%=80(克)
第一次倒出后溶液中含溶质：40×80%=32(克)
加满清水后，溶液浓度为：(80-32)÷100×100%=48%
第二次倒出的溶液中溶质的量为：40×48%=19.2(克)
加满水后，溶液的浓度为(80-32-19.2)÷100×100%=28.8%
第三次倒出的溶液中含溶质的量为：40×28.8%=11.52(克)
加满清水后，溶液的浓度为：(80-32-19.2-11.52)÷100×100%=17.28%。故正确选项为B。

完形填空

Bill, a thirteen-year-old boy, thought he had grown up to be a man. But his parents told him, “You won’t be a real man until you begin to 1  helping others.”
One morning, his parents gave him some money to 2 some milk for them. Outside a shop he saw a homeless old man who looked very sick. Bill went to him and asked, “ What’s wrong with you?”
The old man answered, “I’m 3 . I haven’t had any food for two days.”
At the thought of his parents’ words, Bill said to the old man, “Let’s go to the 4 .”
When they got there, Bill asked the 5 to bring out bread and coffee to the old man. The old man finished the meal quickly. After the waiter 6 the plate and the cup, the old man said, “Sorry for giving you too much 7 . I’m fine now. I’ll   8 forget your kindness! You are a very good young man”
Bill was 9 when he heard this. Just when he wanted to pay for the meal, the waiter came. Bill and the old man learned  10 that he food was free because it was the birthday of the boss, and they were the first customer that day.

A．think about

A．surprisingly

问答题论述题

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