Để răng chắc khỏe và giảm bệnh sâu răng thì hàm lượng flo trong nước cần đạt là 1,0-1,5mg/l . Hãy tính lượng natri florua cần phải pha vào trong nước có sẵn hàm lượng flo từ 0,5mg/l thành nước có hàm lượng flo đạt 1,2mg/l để cung cấp cho 40 triệu dân , mỗi người dùng 250lít nước/ngày. Giả sử natri florua không bị thất thoát trong các quá trình pha trộn và cung cấp đến nơi tiêu dùng.

EXERCISE 1. I.Choose the best answer 1. It is interesting _____________ tree leaves from different countries. A. collecting B. to collect C. collect D. collects 2. My dad enjoys ________ his bike to work. A. to ride B. ride C. rides D. riding 3. I love cooking, _________ my sister doesn’t. A. because B. so C. but D. and 4. Laughing is good _______ your health. A. for B. to C. with D. of Choose the word that has the underlined part pronounced /3:/ A. dear B. pear C. earn D. heart 6. Choose one mistake and correct it. I find carve eggshells boring because it takes a lot of time to complete one shell. A B C D II. Put the verbs in the correct form. 1. I enjoy (fish) .................................................. because it (be) ……………….… relaxing. 2. Jane will play chess after she (finish) …………………………. school. 3. What …………………… your mother often (do) ………………………at weekends? 4. I think, in the future people (not play) ………………………………… individual games. 5. We find (arrange) ……………………………………… flowers interesting because it (help) ………………………………… us relax. 6. I (not collect) ………….……………………… dolls when I grow up. III.Complete the passage with the words given bought competition  singing there  on English Mary’s hobby is singing. She enjoys (1) ………………………… very much. When her brother (2) ………………………… a karaoke set last month, Mary was so glad that she sang every day after dinner. She likes to sing only (3) ………………………… songs. In fact, she has already sung all the songs found (4) ………………………… the karaoke discs. Minh is singing in the karaoke (5) ………………………… organized by the RC Center this Saturday. Her family is going (6) ………………………… to support her. IV.Choose the best answer to complete the sentences by circling A, B, C or D 1. - ............... do you find making pottery? – I finding making pottery interesting. A. What B. How C. Why D. When 2. Jenny...................two eggshells and he will continue the third one. A. carves B. has carved C. carved D. will carved 3. My father hates ………… coffee. He prefers tea. A. to drink B. drink C. drinks D. drinking 4. I have a temperature,………………………………. . A. but I go to bed early B. so I feel tired C. or I am putting on weight D. and I eat more vegetables 5. Which word has the underlined part pronounced differently from the others? A. away B. answer C. sunburn D. picture 6. Which word has main stress differently from the others? A. volunteer B. charity C. melody D. calorie 7. Beethoven ………….. a lot of songs. A. composes B. composed C. has composed D. compose 8. Liz: I am so nervous that I am putting on weight. Tony: ……………………….. . A. Wash your hand more B. Eat less junk food C. Sleep more D. Sunbathe less 9. Jack spends almost his time staring at his smart phone, ……..….. is very short-sighted. A. and B. or C. but D. so 10. My father loved …………. horse-riding when he was young. A. doing B. going C. playing D. taking 11. She feels itchy and her nose is running. She says she has ………….. . A. headache B. toothache C. allergy D. flu 12. Jenifer says that she ……………. collecting dolls, but she…………… in the future. A. will like / won’t continue C. likes / continues B. likes / won’t continue D. will like / continues

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36. The pronunciation course will help you make a big __________ in your spoken English. (IMPROVE) 37. Many __________students choose to study in Australia because of the high quality of education. (NATION) 38. People around the world see the Statue of Liberty as a Symbol of __________. (FREE) 39. Vancouver and Toronto are __________ liveable cities in the world. (FAMOUS) 40. One of the less __________features of California is the threat of earthquakes. (ATTRACT)

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