[問題]OG(11th)DS-Q141

[凱薩琳]

On Jane's cerdit card accoubt, the average daily balance for a 30-day billing cycle is the average of the daily balances at the end of each of the 30 days. At the beginning fo a certain 30-day billing cycle, Jane's cerdit card account had a balance of $600. Jane made a payment of $300 on the account during the nilling cycle. If no other account during the billing cycle, what was the average daily balance on Jane's account for the billing cycle?
(1) Jane's payment was cerdited on the 21st day of the billing cycle.
(2)The average daily balance through the 25th day of the billing cycle was $540.

這題題目看不太懂，也不會作，請會的人指教，謝謝。

[凱薩琳]

Ans:D

[scujean]

見圖......