Hai máy bơm làm việc thì sau 12h bơm nước đầy bể. Nếu máy 1 bơm 3h và máy 2 bơm 18h thì hai máy cũng bơm nước đầy bể .Hỏi mỗi máy bơm 1 mình thì trong bao lâu nước đầy bể?

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9. Forest fires ____________ the pollution of the air. A. make B. result C. cause D. lead 10. The _________ are discussing solutions to the polluted atmosphere in our city. A. pollutants B. environmentalists C. poisons D. fumes 11. She has come __________ with an idea about the environmental project. A. up B. on C. to D. in 12. If the local people drink contaminated water, they _________ have health problems. A. would B. should C. will D. ought to 13. Choose the word that has the underlined part pronounced differently from the others: A. dramatic B. unable C. national D. aquatic 14. Circle a mistake in the following sentence: Water from power stations is able to cause water in rivers hotter. A B C D 15. _______ many households throw garbage on the ground, the land is polluted. A. Because B. So C. Therefore D. Due to 16. Scientists sometimes can't see the stars at night __________ light pollution. A. due to B. because C. so D. since 17. The sight of too many telephone poles, advertising billboards, overhead power lines, or shop signs may cause _________ pollution. A. visual B. thermal C. radioactive D. soil 18. ___________ pollution occurs when the atmosphere contains gases, dust, or fumes in harmful amounts. A. Thermal B. Air C. Radioactive D. Water 19. If people _______ rubbish on the streets, they would look attractive. A. threw B. didn’t throw C. don’t throw D. have thrown 20. __________ pollution is the contamination of lakes, rivers, oceans, or groundwater, usually by human activities. A. Land B. Air C. Radioactive D. Water