某混凝土工程（不受水浸、雨雪作用），混凝土的设计强度等级为C20，设计的坍落度为35~50mm。无前期混凝土强度统计资料。采用32.5MPa级（实测强度为35.7MPa）的普通水泥、细砂、最大粒径为40mm的碎石配制。砂、石子在实验室均为干燥状态，且水泥、砂、石子的密度分别为3.1、2.60、2.65。求C20混凝土的理论（初步）配合比。
参考答案：
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| The students in Shanghai No.3 Girls High School have a flower-arrangement (插花) lesson every week. The course is the 1 of its kind among middle schools in Shanghai. It's a new start. It helps the students to enjoy the 2 of flowers better. That's 3 the course has become very popular among the students. The teachers of the course are all 4 trained. They are good at this art and work hard at passing on the skill 5 students. So far, many stuendts in the school 6 a lot of prizes in this field. In order to further 7 the stuents' skill, the teachers usually play 8 while stuents are arraging flowers. Stuendts 9 the music, then they 10 more good ideas for flower-arrangement. Besids, the teachers teach the stuents other skills at the same time, for example, they ask the students 11 compositions about flower. In the way, students not only get art education but also improve 12 writing skills. Several months ago, some foreigners 13 visited the school. When they saw common flowers become more 14 and meaningful, they got amazed and intereted. They made 15 and would play them for their own studentd after they returned home. | |||
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