Data Sufficiency

How to solve this problem? Do I need to learn about credit card system for it?

IMO D.

If my answer is right then i wil explain.

I agree, it's D

Yup, D. You don't need to know about credit cards, just averages.

Hi

Can u please explain as to why B alone is suff.. I got A as the answer..
I thought B was insufficient since we do not know when the 300$ is getting added to the billing cycle...

Thanks in advance

According to the question the beginning balance of the a/c was 600.

St 1. Jane's payment was credited on the 21st day of the billing cycle.
Which means for the first 20 days the avg was 600 and for the remaining 10 days the bal was 300: (600*20+300*10)/30
St 1 is suff.

St 2. The average daily balance through the 25th day of the billing cycle was $540.
Hence the a/c was credited before the 25th day of the month.
Let X be the number of days the a/c balance was 600. Hence 25-X represents the days the a/c bal was 300.
(600*X+300*(25-X))/25 = 540
Solve for X and find the avg.
St 2 is suff
Answer: D

Thanks. Answer is (D)