Answer to GMAT Question #11

Here is the answer to GMAT Practice Question #11

Answer: D

Explanation:
This problem on the surface may seem confusing, but utilizing the Overlapping Grid is a great way to solve it. Set up a 3 by 3 grid with one type of cancer along the top and the other down the left side:

See Table 1

Then, assuming there are 100 total hospitals, fill in the boxes that you know:

See Table 2

Now, each row must add to the right-most box and each column must add to the lowest box. Knowing that, you can fill in the blanks:

See Table 3

Since the question asks for the number that contains only one type of cancer but not both, we must look at the box for “B but Not A”, which is 40, and “A but Not B”, which is 30. Together, 70 out of 100 hospitals contained one type but not both.

Thus, the probability is 70 percent, or 0.7.