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题库 / GMATLA-DS-12
Joanna bought only $0.15 stamps and $0.29 stamps. How many $0.15 stamps did she buy?
(1) She bought $4.40 worth of stamps.
(2) She bought an equal number of $0.15 stamps and $0.29 stamps.
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
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该题提高速度的方法：可以根据尾数来做。
根据15的倍数和29的倍数，只看个位数，加一起得到0的结果只有2种可能：15取尾数5，同时29取尾数5或者15取0同时29取0
忽略了一个潜在的条件，邮票数x和y都为整，试数
15的倍数个位只可能是0或5；9的倍数就多了0到9都可能
假设15x个位是5，9y的个位就只能是5，y＝5，检验不可能得到x为整；y＝15，已经比440多了，排除
15x的个位只能是0，9y的个位只能0，y不能是0，y＝10只有一种可能
买的个数一定是整数整数！！决定了条件一可以推出来
Let x be the # of $0.15 stamps and y the # of $0.29 stamps. Note that x and y must be an integers. Q: x=?
(1) She bought $4.40 worth of stamps --> 15x+29y=440. Only one integer combination of x and y is possible to satisfy 15x+29y=440: x=10 and y=10. Sufficient.
(2) She bought an equal number of $0.15 stamps and $0.29 stamps --> x=y. Not sufficient.
Answer: A.
邮票的张数只能是正整数n。(1)0.15x+0.29y=4.4,确定x/y是否有一个以上的组合，用个位数为0来试。5的倍数为0或5，9的倍数为0～9，当x为5，y为，5或x为10，y为10时，0.15x+0.29y的尾数为0.试出来两个都为10时等式(1)成立。
答案错了吧
A
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这道题答案应该是A吧？（2）肿么可能充分呢？