OG11-DS141(難...還是不懂....)

Kath1985

90.
On Jane's creditcardaccount, 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 creditcardaccount 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.

Ans
題目說總共欠了$600 還了$300 那麼列式應該為(600-300)除以30呀
為何OG解釋是 [x*600+(30-x)*300]/30  呢?
文章內有句 made a payment是指 又欠了300嗎 ? 可是只有欠了一筆300為何要 300*(30-x )呢? 題目並沒有說欠了很多筆300元呀 ?

Kath1985

所以文章內有句 made a payment是指 又欠了300嗎 ? 可是只有欠了一筆300為何要 300*(30-x) ?  題目並沒有說欠了很多筆300元呀 ?

sissizhx

不对啊,mm,第一天还跟最后一天还，是不一样的，你要把一个月中每天欠费的数额加起来，再除以30，才是arithmetic mean.