虚心求教一道prep数学题

同上问。。。

idon't understand, ask NN's reply too.

we need the average daily balance

according to the question,  the average daily balance ....is the average of the daily balances at the end of each of the 30 days

so we need to know the balance on each of the 30 days

since only 1 transaction happened in the month (-$300)

For condition 1:

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

we know that the daily balance for day 21st then, which is 600-300=300, so for day 22-30, also 300

for day 1-20, 600. so the average= (300x9+600x21)/30= 510

(2) since only 1 transaction happened
and day1st- 25th average is $540, which is less than 600. which means that, the payment is some day before 25th

let the some day be x

so 600x+ 300*(25-x)=540x25

x=20, means, the payment was made on 20th day

the rest of the steps will be similar to condition 1

末了说一句，真TMD的操蛋题

不知道我这么理解对不对

同意楼上，我看完题就选D了。看来各人对题材的敏感度不一样。