[求助]两道prep数学题

huangxiaoyan

six machines, each working at the same constant rate, together can complete a certain job in 12 days.how many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?

答案：3

At a certain college there are twice as many English majors as history majors and three times as many English majors as mathematics majors.What is the ratio of number of history majors to the number of mathematics majors?

答案：3 to 2

坚定之路

第一题，你可以这样想：6台机器工作12天才完成，这样6台机器工作一天完成1/12，单台机器工作一天完成1/72；现在要求8天完成，就是说每天必须完成1/8，需要72/8=9台机器，即需要增加9-6=3台。

第二题，English majors : History majors = 2:1=6:3；English majors : Mathematics majors = 3:1=6:2；可得：History majors : Mathematics majors = 3:2

huangxiaoyan

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