In a sequence...Problem seems to have an error...am I wrong?

[mharney]

This is a data-sufficiency question from the first CAT:

In a sequence of terms in which each term is three times the previous term, what is the fourth term?

(1) The first term is 3.

(2) The second-to-last term is 3^10.

Obviously (1) by itself is sufficient, as the explanation confirms. But (2) with (1) is also sufficient. If you know that the second-to-last term is 3^10, you can calculate all the terms back to the first by dividing by 3 until you reach the first term (3). You'd get this for the whole sequence:

3; 9; 27; 81; 243; 729; 2,187; 6,561; 19,683; 59,049; 177,147

From here, it's easy to see that 81 is the fourth term. To do this, you clearly need (2) AND (1) so that you know what the first term of the sequence is.

But there is no answer choice on a data sufficiency question that says: "(1) is sufficient by itself and (2) is sufficient with (1)"...you're always picking just 1, just 2, both, or neither.

Am I missing something here?

[Ben Ku]

Your question is not about the problem itself, but about attacking Data Sufficiency strategies.

You're right that (1) is sufficient. So the ONLY possibilities are
A - Statement (1) alone is sufficient; statement (2) alone is insufficient, OR
D - Both statements alone are sufficient.

You stated that (2) with (1) is sufficient. This is true, but this also means that (2) ALONE is insufficient. This is exactly A.

Data sufficiency questions are very tricky, so you need to have to know the answer choices and have a solid approach to eliminating answers. In our classes, we teach you to use the AD/BCE grid to help get this straight. If you practice it, it will eliminate the confusion like you had in this problem.