This question illustrates a frequent question type on the GMAT: the “flat fee plus” problem (other common examples include taxi fares and phone plans).
The key with this type of question is to properly account for the flat fee portion in the “plus” calculation. If you aren’t careful, it’s easy to double-count the item that is covered by the flat fee.
To ensure that you count properly,
picking numbers is often a smart strategy on these questions. Here, it’s easiest to pick small integers (aside from the number 1).
Let’s pick x = 2 and y = 3. So we’re getting 3 prints each of 2 different negatives. The first print is $4 and the additional 2 prints are each $1, so the 3 prints for each negative cost $6. Since you have 2 negatives, the total cost is $12.
Now check the answer choices using x = 2 and y = 3 to see what matches $12:
(A) 4xy + (x - 1)y --> 4xy is already 4(2)(3) = $24, so choice A is too big.
(B) 4xy + (y - 1)x --> Also too big for the same reason.
(C) 4x + xy --> 4(2) + (2)(3) = $14
(D) 4x + xy – 1 --> 4(2) + (2)(3) – 1 = $13
(E) 4x + x(y - 1) --> 4(2) + 2(3 - 1) = $12
Only choice E matches, so it is correct.
You could also solve using algebraic translation. The first print for each negative costs $4. Then, we subtract 1 from y to leave us with y – 1 additional prints, each of which costs $1, so the additional prints for each negative cost $(y – 1).
Thus, the cost of all the prints for each negative is 4 + (y – 1). You might be tempted to combine the 4 and the -1, but noting that 4 is in all the answer choices, let’s leave it alone.
Finally, to find out the total cost we multiply the number of negatives x times the cost per negative 4 + (y – 1):
x(4 + (y – 1))
Then distribute the x:
4x + x(y – 1)
This is answer choice E.
Note the trap answer C, which would result if you forgot to subtract 1 from y to account for the fact that the first print was already paid for in the $4 fee. Picking numbers makes this more obvious (without taking a lot of time to calculate), so for most students will be preferable here.
Either way you solve,
it’s critical to account for the initial portion (the first print) in the plus calculation (the additional prints), advice that applies to all “flat fee plus” questions.