Double Matrix method

[Mo2men]

Jack took an exam of 45 questions, how many questions did he answer correctly?

(1) The number he answered incorrectly is twice the number of questions he did not attempt at all.
(2) The total number of questions he answered incorrectly or did not attempt is 3/9 of all questions.

How I can use double matrix method? How statement 2 is interpreted when using 'or'?

I feel it is illogical to say un-attempted question is either correct or incorrect.

Thanks

[GMATGuruNY]

Jack took an exam of 45 questions, how many questions did he answer correctly?

(1) The number he answered incorrectly is twice the number of questions he did not attempt at all.
(2) The total number of questions he answered incorrectly or did not attempt is 3/9 of all questions.

For each question, there are 3 options:
Correct, Incorrect, Not Attempted.

Let C = correct, I = incorrect, N = not attempted.

Since the total number of questions = 45, we get:
C + I + N = 45.

Statement 1:
Case 1: N=1, I=2, C = 45-1-2 = 42.
Case 2: N=2, I=4, C = 45-2-4 = 39.
Since C can be different values, INSUFFICIENT.

Statement 2:
Since I + N constitute 3/9 of the 45 questions, C must constitute 6/9 of the 45 questions:
C = (6/9)45 = 30.
SUFFICIENT.

The correct answer is B.

[GMATGuruNY]

How I can use double matrix method?

Because there is no overlap between correct and incorrect -- a question cannot be answered BOTH correctly and incorrectly -- I would not use a double-matrix.

How statement 2 is interpreted when using 'or'?

incorrect or not attempted = (total incorrect) + (total not attempted).

[Mo2men]

For each question, there are 3 options:
Correct, Incorrect, Not Attempted.

Let C = correct, I = incorrect, N = not attempted.

Since the total number of questions = 45, we get:
C + I + N = 45.

Statement 1:
Case 1: N=1, I=2, C = 45-1-2 = 42.
Case 2: N=2, I=4, C = 45-2-4 = 39.
Since C can be different values, INSUFFICIENT.

Statement 2:
Since I + N constitute 3/9 of the 45 questions, C must constitute 2/9 of the 45 questions:
C = (2/9)45 = 10.
SUFFICIENT.

The correct answer is B.

Thanks for your response.

But i do not understand how you arrive to 'C must constitute 2/9 of the 45 questions'.

I think I+N= 3/9 *45 =15

So C= 30

Can you please clarify??

[GMATGuruNY]

I think I+N= 3/9 *45 =15

So C= 30

Can you please clarify??

Good catch.
My post above has been corrected to read as follows:
Since I + N constitute 3/9 of the 45 questions, C must constitute 6/9 of the 45 questions:
C = (6/9)45 = 30.

[Mo2men]

How statement 2 is interpreted when using 'or'?

incorrect or not attempted = (total incorrect) + (total not attempted).

If we use the word 'and' instead of 'or', does it make any difference?

Thanks for support

[GMATGuruNY]

How statement 2 is interpreted when using 'or'?

incorrect or not attempted = (total incorrect) + (total not attempted).

If we use the word 'and' instead of 'or', does it make any difference?

Thanks for support

For a question to be answered incorrectly, it must be ATTEMPTED.
Thus, it is not possible for a question to be incorrect AND not attempted.

[Mo2men]

For a question to be answered incorrectly, it must be ATTEMPTED.
Thus, it is not possible for a question to be incorrect AND not attempted.

Thanks a lot.

good catch. It is certainly illogical.