GMAT prep software question

atarafder wrote:I can figure out how A is sufficient.

Can you show me how B is sufficient?
Since the average daily balance was $540 at the end of the 25th day, the payment of $300 had been already applied to the balance?
Can a part of the payment be applied after the 25th?

The average can be computed as

[ 600*x + 300*(30-x) ] / 30

1 - sufficient. We know x = 21. So, we can compute the average

2 - sufficient. Payment already applied. You can use the above
formula but replace 30 with 25 and equate it to 540. You can solve
for x and then compute the average for 30 days.

Hence D.

great. Thanks!