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by Neilsheth2 » Tue May 24, 2016 11:51 pm
Hello Experts ,

Could you please help me understand why is the answer D?
My approach:-
For A-> 21/0.07 =300 minutes so sufficient
For B->40-21(since first 300 minutes and then the rest must have gone as per 0.05) so sufficient
Therefore D

Pls help me. Am I missing the big picture?

Thank you in advance.
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Source: — Data Sufficiency |
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by GMATGuruNY » Wed May 25, 2016 4:59 am
Neilsheth2 wrote:Hello Experts ,

Could you please help me understand why is the answer D?
My approach:-
For A-> 21/0.07 =300 minutes so sufficient
For B->40-21(since first 300 minutes and then the rest must have gone as per 0.05) so sufficient
Therefore D

Pls help me. Am I missing the big picture?

Thank you in advance.
Statement 1:
For the number of minutes charged at a rate of 7 cents per minute, Bob was charged a total of $21.
Thus:
Number of minutes charged at a rate of 7 cents per minute = (total charge at 7 cents per minute)/(rate per minute) = 21/(.07) = 2100/7 = 300 minutes.
No information about how many minutes were charged at a rate of 5 cents per minute.
It's possible that 0 MINUTES were charged at a rate of 5 cents per minute, with the result that the total time = 300 minutes.
It's possible that 100 MINUTES were charged at a rate of 5 cents per minute, with the result that the total time = (minutes charged at 7 cents per minute) + (minutes charged at 5 cents per minute) = 300+100 = 400 minutes.
Since the total time can be different values, INSUFFICIENT.

Statement 2:
For the number of minutes charged at a rate of 5 cents per minute, Bob was charged a total of $40.
Thus:
Number of minutes charged at a rate of 5 cents per minute = (total charge at 5 cents per minute)/(rate per minute) = 40/(.05) = 4000/5 = 800 minutes.
Since the rate decreases to 5 cents per minute only after 300 minutes have been charged at a rate of 7 cents per minute, the number of minutes charged at a rate of 7 cents per minute = 300 minutes.
Thus:
Total time = (minutes charged at 7 cents per minute) + (minutes charged at 5 cents per minute) = 300+800 = 1100 minutes.
SUFFICIENT.

The correct answer is B.
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by jain2016 » Wed May 25, 2016 9:03 am
Hi GMATGuruNY ,

How come 1st statement is insufficient and 2nd statement is sufficient? I really dont understand.

Can you please help in this.

Many thanks in advance.

SJ
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by [email protected] » Wed May 25, 2016 9:49 am
Hi Neilsheth2 and jain2016,

You have to be careful about how you interpret the information in this prompt. Before we get to the two Facts, it's important to understand the information that we're given to start with.

The phone company charges $0.07 per minute for the first 300 minutes of use and $0.05 per minute for each ADDITIONAL minute above 300 minutes.

This means that if you use the phone for 300 minutes, then that will cost you a total of ($0.07)(300) = $21. If you use the phone for 301 minutes, then you're still charged $21 for the first 300 minutes and then you're charged another $0.05 for the extra minute... for a total of $21.05. Any total above 300 minutes will have $21 of charges at $0.07 a minute AND an extra charge at $0.05 for each additional minute.

We're asked for the total number of minutes that Bob was charged.

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

This Fact proves that Bob used AT LEAST 300 minutes of calling time. He might have used 301 minutes, 302 minutes, etc. though, but we aren't told how much money he was charged for THAT EXTRA time (only the time that was charged at $0.07 per minute).
Fact 1 is INSUFFICIENT.

2) Last month the phone company charged Bob $40.00 for his calling time that was charged at $0.05 per minute.

With this Fact, we know that Bob MUST have already used 300 minutes at $0.07 a minute... THEN the extra minutes (at $0.05 a minute) were added on (with a total of $40.00 charged for THOSE minutes. Those extra minutes would total (40)(20) = 800 minutes.... in addition to the 300 minutes he would have had to have used at $0.07 a minute.
Fact 2 is SUFFICIENT.

Final Answer: B

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by jain2016 » Wed May 25, 2016 9:32 pm
1) Last month the phone company charged Bob $21.00 for his calling time that was charged at $0.07 per minute.

This Fact proves that Bob used AT LEAST 300 minutes of calling time. He might have used 301 minutes, 302 minutes, etc. though, but we aren't told how much money he was charged for THAT EXTRA time (only the time that was charged at $0.07 per minute).
Fact 1 is INSUFFICIENT.
Hi Rich ,

Thanks for your reply. Just a quick clarification.

In statement 1 it is given that bob was charged $21. So the minutes will 21/0.07 = 300 minutes that means there is no additional minutes.

Why not like above?

Please explain.

Many thanks in advance.

SJ
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by [email protected] » Thu May 26, 2016 9:35 am
Hi jain2016,

In my prior post, I mentioned how you have to be really careful about how you interpret the information in this prompt. Fact 1 tells us that the phone company charged Bob $21.00 for his calling TIME that was charged at $0.07 per minute. It does NOT state that he was charged a total of $21 - it just tells us what he was charged for at that $0.07 a minute rate. This Fact proves that Bob used AT LEAST 300 minutes of calling time. He might have used 301 minutes, 302 minutes, etc. though. (but those extra minutes are charged at $0.05 a minute, so they would not be included in the 'group' of minutes that were charged at $0.07 a minute).

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by Matt@VeritasPrep » Thu May 26, 2016 3:04 pm
jain2016 wrote:Hi GMATGuruNY ,

How come 1st statement is insufficient and 2nd statement is sufficient? I really dont understand.

Can you please help in this.

Many thanks in advance.

SJ
The problem with A is that it doesn't tell us anything, other than that we used all of our first 300 minutes.

Suppose we talk for m minutes. We know the first 300 minutes cost 7¢ each, and the remaining (m - 300) cost 5¢ each. That gives us a cost of

.07*300 + .05*(m - 300)

So the first 300 minutes cost $21 no matter what. (You could use less than 300 minutes, of course, in which case the equation is just .07 * (# of minutes), but each statement tells us this scenario is impossible.)
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