Last month a phone company charged $ 0.07 per minute for the first 300

C for me

1)gives time for .07 minute call .he talked for amount 21$=300 minutes.not suff(no inf about 0.05 call)
2) 40$ for 0.05 call i.e 800 minutes.not suff(no info about 0.07 call)

C-total of 1100 minutes

IMO, Answer must be B.

1) Last month the phone company charged Bob $21.00 for his calling time that was charged at $0.07 per minute.

This statement says Bob made calls for 300 mins for which he was charged $21. But it doesn't tell if Bob called only for 300 minutes or more.

not sufficient.

2) Bob was charged $40 at the rate 0.05/minute.

So the minutes he was charged at this rate were= 40/.05= 800 minutes

But, since this rate is over 300 minutes. So the total minutes he was charged for were= 800+300= 1100 minutes

Sufficient.

I have a question about answer choice B and any help on this question would be much appreciated. B states that Bob was charged 40 bucks at 5 cents per minute, so his total minutes at that rate amounts to 800. Ok, got it so far. What I don't get is that under this statement, it is still possible for the number of minutes at the 7 cent/min rate to vary. For example, couldn't Bob have made one giant 1100 minute call, where he stays on the phone 800 mins past the initial 300, as well as a 300 minute call, then he stays on the phone for 1 minute, but does this a whole bunch of times? In other words, he could have made a 300+ minute call, then stayed on the phone for some x amount of time, let's say he accrued $20 in charges. Then he makes another 300+ min call, accruing another $20; such scenario would lead to multiple total minutes. Please help!

EDIT: Okay nevermind, I see now why the answer is B...
I misinterpreted the question as saying that anytime you exceed 300 mins of calling a new rate kicks in...and not that the new rate is rather applied in terms of total minutes...which totally makes sense because what kind of company designs a plan where the rate changes after 300 mins on the phone...

Still, when you're speeding through the GMAT and your mind is in critical/nit-pick mode for every single detail, it's easy to construe the words in such way. The question as written can easily be read to say that the 7 cent rate applies to the first 300 mins of calling time, and the new rate kicks in after.

It wouldn't hurt if the question added the word total calling time as oppose to per call time so to make the difference, as I don't think a business mind should be on the look out for such misinterpretations.

2) Because 2 tells you the cost of the calls charged at $0.05/min, you can deduce that the first 300 minutes of calling time at $0.07/min was used up. Therefore, 2) is suff. B

chetan2u

i have little difficulty understanding how B is correct

how do you know $40 was counted over a single phone call
option B states the charge @0.05 for the entire month
so there might be chances he used 2*$20= $40 for entire month
then total may be = 300+ 300+ 800= 1400

Please explain experts !!!!!!!

Hi, The best way to solve this question is to set the appropriate equation.

Here, let X = the total time spent on the phone.

Thus you have the equation:

TOTAL CHARGE = .07(300) + .05(X-300) --> TOTAL CHARGE = 21 + .05X - 15
Where the $21 derives from the .07 cost per minute. And the .05x - 15 derives from the .05 cost per minute after the 300.

Statement 1
Last month the phone company charged Bob $21.00 for his calling time that was charged at $0.07 per minute.
Insufficient because we don't know about the second part of the charge, and additionally, this is information we already know.

Statement 2
Last month the phone company charged Bob $40.00 for his calling time that was charged at $0.05 per minute.
Sufficient because this tells us that .05X - 15 = 40. Therefore we can solve for X and get the answer.

B.

Aren't both sufficient: From 1) we deduce that Bob was charged for 300 minutes only for the month; and from 2) 1100 minutes for the month?

A follow up question: in cases where both statements are sufficient, do they usually have to give the same answer? (e.g. this could be a way to sense check your answer I imagine)

chetan2u
gmatbusters
VeritasKarishma

When I attempted this question, I went for option D.
The question does not explicitly state whether Bob Used the services for 300 or more than 300 minutes.
SO when evaluating statement 1 , I thought that the total time for talking was only 300 minutes.
Hence I chose option D.
What am I missing in this question?
Is there a way I could ignore these errors in the future?

Regards

This is what the question is: For how many minutes of calling time was Bob charged by the phone company last month?

You can't assume 300 mins and then go in the statements. We have to find for how many mins he was charged. He could have been charged for 100 mins or 500 mins!
Stmnt 1 gives no new info. The question stem tells us that "company charged $ 0.07 per minute for the first 300 minutes of calling time"
So total cost for first 300 mins is .07*300 = $21
Stmnt 1 tells us nothing new.
We don't how much the company charged him for $0.05 per min calls.

Given: Last month a phone company charged $ 0.07 per minute for the first 300 minutes of calling time and $0.05 per minute for each minute of calling time in excess of the first 300 minutes .

Target question: For how many minutes of calling time was Bob charged by the phone company last month?

Statement 1: Last month the phone company charged Bob $21.00 for his calling time that was charged at $0.07 per minute.
$21.00/$0.07 = 300 minutes
So, Bob used up all of this 300 minutes at $0.07/minute
HOWEVER, we don't know how many minutes he used BEYOND the first 300 minutes.
Here are two possible scenarios that satisfy statement 1:
Case a: Bob used 300 minutes at $0.07/min and 100 minutes at $0.05/min. In this case, the answer to the target question is Bob was charged for a total of 400 minutes of calling
Case b: Bob used 300 minutes at $0.07/min and 200 minutes at $0.05/min. In this case, the answer to the target question is Bob was charged for a total of 500 minutes of calling
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: Last month the phone company charged Bob $40.00 for his calling time that was charged at $0.05 per minute.
IMPORTANT: the $0.05 per minute only comes into effect once one has used all 300 minutes (at $0.07 per minute)
So, statement 2 indirectly tells us that Bob used up all 300 minutes (at $0.07 per minute)
$40.00/$0.05 = 800 minutes (at $0.05 per minute)
The answer to the target question is Bob was charged for a total of 1100 minutes of calling
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer: B

Cheers,
Brent

this is a GMAT Prep question. please tag it.

_______________________
Done. Thank you.

Imo this is the best explanation out of all of them. Thanks

Bunuel need your help in this one

Please tell why A is insufficient

The wording here is definetly ambigious.
You are all right in saying we do not have any information going in.
People like me who have marked D are simply relying on information given in the DS.
Here is the question it raises:
Can we know for a fact that he did not speak exactly 300 minutes, hence causing the bill to be exactly $21.
If we look (1) and (2) in isolation, as is the point of D , we find that (1) satisfies the problem because he was charged $21. Why assume that $21 is an incomplete charge?
If the statement is sufficiently telling me he was charged $21 for the month. If the math works out, why should I not see that as a right answer as well.
(2) in isolation also gives a similarly easy answer.

Ru8ix123, the response maybe a little late but as this is a GMAT prep question, this post might help someone down the line.

I'll address your highlighted portion. We're not assuming, we're just saying we don't know if it is complete. You cannot assume the charge to be incomplete, but you also cannot assume it to be complete. As the statement specifically says "$21.00 for his calling time that was charged at the rate of $0.07 per minute." You only have the charge for one of the rates. Of course, if the statement said that the total charge was $21, it would have been enough.

As is the case with every DS question, there should be no wiggle room for a statement to be correct.
Also, another thing to note is that whenever the answer is "D". Both the statements should give the same answer.