One approach is to use the Double Matrix Method. This technique can be used for questions featuring a population in which each member has two characteristics associated with it (aka overlapping sets questions).
Here, we have a population of vehicles, and the two characteristics are:
- truck or not a truck
- functional or non-functional

Aside: We can also use Venn diagrams and formulae to solve overlapping sets questions. However, as difficulty levels increase, it becomes harder to apply those other approaches, whereas the Double Matrix Method works every time.

Since we are given information the form of percentages, and since the question asked us to find a certain percent, let's assign a nice value to the number of vehicles in the parking lot.
Let's say there are 100 vehicles in total.
If 36% of the vehicles are non-functional trucks, then the number of non-functioning trucks = 36% of 100 = 36.

Let's add this information to our diagram:


Given: 40% of the non-functional vehicles are not trucks
Since we don't know the total number of non-functioning vehicles, let's let x = the total number of non-functioning vehicles
This means that 0.4x = the number of non-functional vehicles that are not trucks
Add this information to our diagram to get:


Since the two boxes in the right-hand column must add to x, we can write: 36 + 0.4x = x
Subtract 0.4x from both sides of the equation: 36 = 0.6x
Solve: x = 36/0.6 = 360/6 = 60
This means that 60 the 100 vehicles are non-functional, which tells us the remaining 40 vehicles are functional

Answer: 40


This question type is very common on the GRE, so be sure to master the technique.

To learn more about the Double Matrix Method, watch this video:


Then you can try solving these two questions:
- https://gre.myprepclub.com/forum/qotd-11-o ... -2660.html
- https://gre.myprepclub.com/forum/on-a-cert ... 13470.html