OG 242 看不懂题目!!

242. 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 sycle. If no other amounts were added to or substracted 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

答案是D

不过题目我不太了解它说舍么  有人可以讲解一下吗!!  谢谢 !!

在jane的信用卡帐户, 30天的billing周期内的平均值是每天结束时的帐面欠帐的平均.在某个30天周期的开始,jane帐户上有600$欠帐, jane在这个周期内的某天付了300.如果没有其他帐面转入或转出(就是没再加钱, 也没再减去),jane的帐户的每天平均欠帐是多少

balance:欠帐(我的理解)

242. 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 sycle. If no other amounts were added to or substracted 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

so want to know the average daily balance on jane's account for the billing cycle, in other words, we want to know the arithmetic mean of the balance on jane's account throughout the month (balance on day 1 + balance on day 2 + ... + blance on day 30) / 30, so in order to get the average, we need information on the balances throughout the month, we know that $600 dollars was the balance in th e beginning of the month, and $300 was put in some time during the month and was the only transaction, now:

(1)Jane's payment was credited on the 21st day of the billing cycle.

if jane's payment was credited on the 21st day of the billing cycle then the balance of the first 20 days would be $600 and for the last 10 days would be 600 - 300 = 300, thus the mean is (600 * 20 + 300 * 10) / 30 = 500, thus 1) by itself is sufficient

(2)The average daily balance through the 25th day of the billing cycle was $540.

if the average daily balance through the 25th day of the billing cycle was 540, that means 300 has been put in already for the average to be lowered than 600, so (600 * x + 300 * y) / 25 = 540, x + y = 25 => x = 20, and y = 5, and thus we know the balance through the first 20 days is 600, thus 2) is sufficient to know the answer

thus the answer is D

thanks  horsefish and tiger1234

完全理解了  的确是balance单字理解的问题

balance (AMOUNT)   [Show phonetics]
noun [C usually singular]
the amount of money you have in a bank account, or the amount of something that you have left after you have spent or used up the rest:
Once we know how much money we'll need, let's spend the balance (= the amount left).
The company's success is reflected in its healthy bank balance.

horsefish 的理解"欠帐"  很好啊

也感谢 tiger1234 比 OG  还详细的解释  本来正想确认算式里的300是不是 由600-300 得来的 看了一下 里面就有解答了