※ 引述《rnrn ()》之銘言： : 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. : 答案是(D) 請幫我解答 謝謝^^ 選項一 300塊在第21天入賬 所以第一到第20天的balance是600,第21天到第30天的balance是300 平均就可以算的出來 選項二 設balance為600的天數為x,balance為300的天數為y 解聯立 (600x+300y)/25=540 x+y=25 得出x=20, y=5 平均也可以算的出來 以上 --

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