Data Sufficiency

nandy1984 wrote:On Jane's credit card account, 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 day. 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) Average daily balance through the 25th day of the billing cycle was $540

Please so explain how to solve them within 2 mins...Thanks

Explanation:
We need to find the average daily balance on Jane's account for the whole month. It is given in the question that no other amounts were added to or subtracted from the account during the billing cycle, so the balance remains unchanged, that is $300, for the remaining month. This means that we can reword the question as "On which day did Jane make the payment?"

Statement 1 directly gives the answer to the question, which is the 21st day of the billing cycle; SUFFICIENT.

Statement 2: Let us assume that Jane makes the payment on Nth day.
Then for N - 1 days, the balance was $600.
For 25 - (N - 1) or 26 - N days, the balance was $300.
The average balance = [(N - 1)600 + (26 - N)300]/25 = 540, which can be solved to find the value of N; hence SUFFICIENT.

The correct answer is D.