Hi oquiella,

The prompt that you listed was missing some information. The full prompt is here:

Last month a certain music club offered a discount to preferred customers. After the first compact disc purchased, preferred customers paid $3.99 for each additional compact disc purchased. If a preferred customer purchased a total of 6 compact discs and paid $15.95 for the first compact disc, then the dollar amount that the customer paid for the 6 compact discs is equivalent to which of the following?

(A) 5(4.00) + 15.90
(B) 5(4.00) + 15.95
(C) 5(4.00) + 16.00
(D) 5(4.00 - 0.01) + 15.90
(E) 5(4.00 - 0.05) + 15.95

Like many GMAT questions, this prompt isn't really about doing lots of math - it's about organizing the information that you're given so that you can avoid doing lots of calculations.

Notice how the first disc is $15.95? That's $0.05 LESS than $16.
Notice how the additional discs are all $3.99? That's $0.01 LESS than $4.

It's interesting that the question tells us that after the first disc is purchased, 5 additional discs are purchases. So that would be $0.05 less than $20. Thinking in these terms ($0.05 'pieces'), the numbers in this prompt are actually pretty easy to combine and simplify.

GMAT assassins aren't born, they're made,
Rich

oquiella wrote:Last month a certain music club offerred a discount to preferred customers. After the first compact disc purchased, preferred customers paid $3.99 for each additional compact disc purchased. If a preferred customer purchased a total of 6 compact disc, then the dollar amount that the customer paid for the 6 compact discs is equivalent to which of the following?

A. 5(4.00) + 15.90
B. 5(4.00) + 15.95
C. 5(4.00) + 16.00
D. 5(4.00-0.01) + 15.90
E. 5(4.00-0.05) + 15.95

The preferred customer paid 15.95 + 5(3.99) dollars for the 6 CDs. However, we don't see this expression in the answer choices. We can see that had the 15.90 in choice D been 15.95, then D would be the correct answer. Or had the 0.05 in choice E been 0.01, then E would be the correct answer. However, since they aren't, neither one is the correct answer.

Now let's analyze the remaining three answer choices. A, B and C. Notice that each one has 4.00, which is 0.01 more than 3.99 and the difference between 5(4.00) and 5(3.99) is 5(0.01) = 0.05. Thus we see that, since 5(4.00) overvalued the cost by 0.05, we must undervalue 15.95 by 0.05 to 15.90 to have an equivalent cost. Thus, equivalently, the customer paid 5(4.00) + 15.90 dollars.

Answer: A