求PREP-DS-2-109 解

jesuisdesole

109.
                11067-!-item-!-187;#058&007824

Marta bought several pencils.  If each pencil was either a 23-cent pencil or a 21-cent pencil, how many 23-cent pencils did Marta buy?

 

(1)  Marta bought a total of 6 pencils.

 

(2)  The total value of the pencils Marta bought was 130 cents.

答案是B

求思路,谢谢

bellamama

第二个条件单独可以成立，130分可以有23，23，21，21，21，21组成，而且只能由这几个数组合在一起才正好是130。

xinxian

(1) x+y=6 (Insufficient to get 'y')
(2) 21x+23y=130 (Sufficient because, when you substitute whole numbers for x & y, you'll get only one correct match i.e., x=4 & y=2)