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题库 / Prep2007E1-DS-90
On Jane's credit card account, the average daily balance for a 30-day billing cycle is the average (arithmetic mean) of the daily balances at the end of each of the 30 days. At the beginning of a certain 30-day billing cycle, Jane's credit card account had a balance of $600. Jane made a payment of $300 on the account during the billing cycle. If no other amounts were added to or subtracted from the account during the billing cycle, what was the average daily balance on Jane's account for the billing cycle?
(1) Jane's payment was credited on the 21st day of the billing cycle.
(2) The average daily balance through the 25th day of the billing cycle was $540.
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.
算一个30天内帐户余额的平均数。
告诉你在最初时候帐户里有600块，然后这三十天中帐户唯一的变化就是某天用掉了300。
条件1就他说那个钱是第21天付的，那就直接（20×600＋10×300)/30。就是说前20天帐户里都是600块，后十天都是300块，算个平均就行了。
条件2说前25天的平均数是540。那说明那300块肯定是在前25天中的某天中付的，否则平均数应该是600。设 有x天用300；x 300+(25-x)600=25乘540 x=5 最后求出( 5*300+25*600 )/30
两个条件都能算。选d, either。
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