晕：问2道PREP数学

gutou · 发表于 2011-1-6 20:06:52

搞不明白，请教头脑清醒的~
1，DS. One Kilogram coffee consists of x kilo type I and y kilo tyep II. The cost of the coffee can be represented as C=6.5x+8.5y. Is x<0.8?
1) y>0.15
2) C>=7.3
答案选B.不懂。我怎么算不出来啊？

2， A car, on average it can be driven 25 miles for one gallon fuel in city and 40 miles for one gallan fuel on highway. What is the closest number of miles (average) if the car is driven 10 miles in city and 50 miles on highway? 没思路。能算出来用了多少油，但是怎么算average mile呢？
头很晕啊，看不进去什么，强撑着呢

sdcar2010 · 发表于 2011-1-6 22:56:01

1，DS. One Kilogram coffee consists of x kilo type I and y kilo tyep II. The cost of the coffee can be represented as C=6.5x+8.5y. Is x<0.8?
1) y>0.15
2) C>=7.3
答案选B.不懂。我怎么算不出来啊？

A hidden condition is y=1-x. Substitute y with 1-x in the cost equation, you will have an easier time to solve this question.

sdcar2010 · 发表于 2011-1-6 22:59:01

2， A car, on average it can be driven 25 miles for one gallon fuel in city and 40 miles for one gallan fuel on highway. What is the closest number of miles (average) if the car is driven 10 miles in city and 50 miles on highway? 没思路。能算出来用了多少油，但是怎么算average mile呢？

Reverse engineering.

Use the info above to get the gallons of gas on average will be consumed if driven 10 miles in city and 50 miles on highway. Then use that number to devide (10+50) miles . . .

lonelyorchid · 发表于 2011-1-6 23:19:35

第一题:
因为x+y=1,所以 C=6.5+2y or C=6.5-2x
如果C〉=7.3 那么得出 x=《 0.6

第二题:
根据条件，可以:
In city 1/25 galon per mile
On highway 1/40 galon per mile
1.6 galon per (10 miles in city and 50 miles)
混合路况平均油耗 1galon per 37.5

gutou · 发表于 2011-1-7 06:54:27

原来如此！
1， X+Y=1我看出来了，昨天晕的很愣是算不出答案！
2，哦，原来是除（10+50）啊。。。没搞懂~
谢谢两位！

晕：问2道PREP数学

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