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?"sud21 wrote:
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.
