To rent a tractor, it costs a total of x dollars for the first 24 hours, plus

[BTGModeratorVI]

To rent a tractor, it costs a total of x dollars for the first 24 hours, plus y dollars per hour for each hour in excess of 24 hours. Which of the following represents the cost, in dollars, to rent a tractor for 36 hours?

(A) x + 12y
(B) x + 36y
(C) 12x + y
(D) 24x + 12y
(E) 24x + 36y

Answer: A
Source: Official guide

[Brent@GMATPrepNow]

To rent a tractor, it costs a total of x dollars for the first 24 hours, plus y dollars per hour for each hour in excess of 24 hours. Which of the following represents the cost, in dollars, to rent a tractor for 36 hours?

(A) x + 12y
(B) x + 36y
(C) 12x + y
(D) 24x + 12y
(E) 24x + 36y

Answer: A
Source: Official guide

These kinds of questions (Variables in the Answer Choices - VIACs) can be answered algebraically or using the INPUT-OUTPUT approach.
The other posters above have solved the question algebraically, so let's use the INPUT-OUTPUT approach.

First, choose values for x and y
Let's say x = 4 and y = 3

In other words, to rent a tractor, it costs a total of 4 dollars for the first 24 hours, plus 3 dollars per hour for each hour in excess of 24 hours. Which of the following represents the cost, in dollars, to rent a tractor for 36 hours?

TOTAL cost = 4 dollars (for the first time for hours) + (12)(3 dollars) (for each of the 12 hours after the first 24 hours)
= 4 + 36
= 40 (dollars)

So, when x = 4 and y = 3, the answer to the question is 40 (dollars)
In other words, when we INPUT x = 4 and y = 3, the OUTPUT is 40

We'll now take each answer choice and plug-in x = 4 and y = 3.
The correct answer choice will be the one that evaluates to be 40

(A) 4 + 12(3) = 40 Great!
(B) 4 + 36(3) = 112. No good. We need an output of 40
(C) 12(4) + 3 = 51. No good. We need an output of 40
(D) 24(4) + 12(3) = 132. No good. We need an output of 40
(E) 24(4) + 36(3) = 204. No good. We need an output of 40

Answer: A

Cheers,
Brent

[Scott@TargetTestPrep]

To rent a tractor, it costs a total of x dollars for the first 24 hours, plus y dollars per hour for each hour in excess of 24 hours. Which of the following represents the cost, in dollars, to rent a tractor for 36 hours?

(A) x + 12y
(B) x + 36y
(C) 12x + y
(D) 24x + 12y
(E) 24x + 36y

Answer: A
Source: Official guide

Solution:

The cost to rent the tractor for 36 hours is:

x + y(36 - 24) = x + 12y

Answer: A