问一道数学题

miguel29

50.4230-!-item-!-187;#058&003784

A construction company was paid a total of $500,000 for a construction project.The company's only costs for the project were for labor and materials.Was the company's profit for the project greater than $150,000 ?



(1)The company's total cost was three times its cost for materials.



(2)The company's profit was greater than its cost for labor.

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【答案】C

【思路】profit= total – (labor+ material)問profit>150000

(1) (l+m)=3m-------------------------------------------------------------------insufficient

(2)p>l---------------------------------------------------------------------------- insufficient

(1)(2)p=500,000-(l+m), p=500000- (l+
l), P=500000-1.5l
數字代入法P=15(千), l=23(千) ; P=25(千),l=16(千)-------------------------- sufficient

我数学很差，看了这个解释还不是怎么明白为什么这题选C，为什么它最后可以用数字代入法就可以知道这个利润是>15000的？不是最后推论到那里还是有一个L不知道嘛？怎么一用数字代入法就能确定它的利润>15000了呢？