哪位好心人教教我怎么做！

RobinSW

A factory worker produced one lot of connecting clips, and initial quality control found that 5/7 of clips in the lot could be used. An assembly plant purchased this lot of clips valuing the lot at $3 per each usable clip. During assembly, it became known that only 3/5 of the clips in the lot were usable. What was price per usable clip that the assembly plant paid for the lot?                                    
                                    

• A 2 1/7
• B 3 4/7
• C 5 1/4
• D 5
• E 6 3/8
  

A的意思是 2又7分之1


正确答案  B  

pattyx

假设一个lot是X个，其中5/7*X是可用的，每个$3，也就是付了$15X/7

然后发现只有3/5*X是可用的，也就是3X/5个

总价/数量= (15X/7)÷(3X/5)=25/7=3又4/7

RobinSW

明白了，谢谢美女