Last month a phone company charged \(\$0.07\) per minute for the first \(300\) minutes of calling time and \(\$0.05\)

[Gmat_mission]

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

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

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

Answer: B

Source: GMAT Prep

[Brent@GMATPrepNow]

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

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

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

Answer: B

Source: GMAT Prep

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