Hey Silvia,

So, we know that Jane starts the month with $600 in the account, and we know she makes a payment of $300. We're also told that there are no other transactions. So, that means the end-of-the-day balance for every single day in this month has to be either $600 or $300.

The problem is, we don't know how many $600 days there are and how many $300 days there are. We need to get information on when the $300 was deducted. Once we know that, we know exactly how many of each day there were, and thus, we can find the average for the whole month.

Statement (1) gives us the exact day on which we go from $600 days to $300 days, so we know exactly how many of each day there were. SUFFICIENT

Statement (2) says that the average through the 25th day was less than $600. That means the switch to $300 days has already taken place, which means that the remaining days in the month are all $300 days. Since we know the average of the first 25 days, and since we know the rest of the days are all $300 days, we can thus find the average of the whole month. SUFFICIENT

Make sense? Let me know if anything needs clarification!

raz1024 wrote:Hey Silvia,

So, we know that Jane starts the month with $600 in the account, and we know she makes a payment of $300. We're also told that there are no other transactions. So, that means the end-of-the-day balance for every single day in this month has to be either $600 or $300.

The problem is, we don't know how many $600 days there are and how many $300 days there are. We need to get information on when the $300 was deducted. Once we know that, we know exactly how many of each day there were, and thus, we can find the average for the whole month.

Statement (1) gives us the exact day on which we go from $600 days to $300 days, so we know exactly how many of each day there were. SUFFICIENT

Statement (2) says that the average through the 25th day was less than $600. That means the switch to $300 days has already taken place, which means that the remaining days in the month are all $300 days. Since we know the average of the first 25 days, and since we know the rest of the days are all $300 days, we can thus find the average of the whole month. SUFFICIENT

Make sense? Let me know if anything needs clarification!

Hi Raz_1k:

To me, the first line is a reading comprehension problem: "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 days". It should say cummulative ! ...I assumed sum(30day$)/30 ...

Stm1 is clear that is Suf.
Stm2: With your explanation that 540 is daily average cumulative from day 1 until day 25th, I don't need to know when the payment of 300 was done, and I can get the avg for day 26, 27 ...

English is not my native language, so understanding this kind of things is not easy to me, but now, it makes sense.
Thanks for your time Raz.

Silvia

Absolutely...difficult DS problems are, in a way, Reading Comp problems.

But notice that for many of them (including this one), you don't need to do any calculations. The GMAT is more interested in testing your knowledge of concepts than in testing your ability to do random calculations.

The trick to a problem like this is figuring out exactly what you need conceptually to arrive at an answer. Everything else (including the numbers, in many cases) is extraneous.