Thanks Brent !

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


1:

now for first 20 days we have 600 as the average
for next 10 days it would be 300

(600*20 + 300*10 )/ 30 = AVERAGE

Suff..

2:
Now let x days we have 600 as balance
y days we have 300 as balance

x + y = 25
600x + 300y / x+y = 540
Solve and find x and y

Suff

IMO D