Hallie has only nickels, dimes and quarters in her pocket. If she has

The equation may be written as 0.05N+0.1D+0.25Q=2.75
St1
N+D+Q=21
D-2N
We have 3 equations for 3 variables which can be solved. Sufficient

St2
0.05N+0.1D=1.25
This may be simplified to
5N+10D=125, or
N+2D=25
This equation may have many solutions. Insufficient.

Answer A

KAPLAN OFFICIAL SOLUTION:

Now this question is certainly answerable. You might have to play with the information, do a little substitution, a little combination, but eventually you’d figure out that she had 5 nickels. Now what do you do with that number? There is no answer choice for 5. There is no way to enter your discovery at all. Strictly speaking the information is useless.

Furthermore, if you focused on answering the question about Hallie’s nickels instead of the data sufficiency question that is being asked, chances are that you would use all the information given in both statements instead of considering them separately. (The problem is easier to solve if you do.) In that case, you will perhaps conclude that the statements taken together are sufficient to answer the question, when in fact, statement 1 by itself is sufficient.

Lastly, this whole process of solving for the number of nickels takes a lot of time-- time you probably wish you could use on other problems.

So how would a “lazy person” approach this question? The lazy person would never just dive in and start sifting information, but would take a bird’s-eye view. He or she would look for the concept. Data Sufficiency is concept math.

Our lazy student would look at the question stem and see that there are three unknowns (nickels, dimes and quarters), that they total up to a definite amount and that he is being asked for the value of one of them separately. That would strongly suggest a system of equations.

Next our lazy student would recall the rule for systems of equations, which is that you must have as many distinct equations as you have variables in order to solve for the individual variables. How many equations does the stem give us? One, for the total dollars. So we need two more.

Statement 1 gives us two more. That there are 21 coins would give us one equation, and that there are twice as many dimes as nickels would give us the other. (Note, an equation to be useful does not have to have all the variables in it, so the lack of quarters in the second equation doesn’t matter.) Together with the stem, that makes three equations, which allow us to solve the system (which we needn’t bother to do).

Statement 2 only gives us one more equation, so it is not sufficient, and the answer is Statement 1 alone is sufficient.
Notice that our lazy student did not even bother to translate the words into formal equations. It’s enough to know that you COULD do so.

So if you find yourself tempted to do extensive mathematical calculations on a GMAT Data Sufficiency problem—remember the “lazy student”, and that the secret on Data Sufficiency problems is that the answer to the mathematical question doesn’t matter in most instances. Keep your eye on the concept being tested rather than the details of the execution.

~By Guest Author, Kurt Keefner

I can't figure out how the answer is option a. Can someone post a detailed explanation?

Detailed explanations are posted above. Please read the thread.

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