# GMAT Probability, Simplified.

Probability questions on the GMAT often confuse students. You need a simple strategy.

Consider the following question:

A canoe has two oars, left and right. Each oar either works or breaks. The failure or non-failure of each oar is independent of the failure or non-failure of the other. You can still row the canoe with one oar. The probability that the left oar works is . The probability that the right oar works is also . What is the probability that you can still row the canoe?

A)

B)

C)

D)

E)

## 1. The wrong way

The temptation is to multiply the two probabilities given to reach the answer . Whenever you get to an answer choice very quickly, particularly when that answer is A, I would look at the question again! Answer choice A is the first answer you see. If you are in a hurry and option A looks right, many test takers will go for A.

- This calculation only gives you the probability that both oars work.
- To get the right answer, you would also have to add the probability that the left oar works and the right fails.
- Then you would also have to add the probability that the right works, but the left fails.

All this would be possible, but slow. Is there a better way? Yes!

## 2. The right way

Simply look at the question from the other side. What is the probability that you can’t row the canoe? This would be .

Using the idea that the probability of something happening is the probability that it doesn’t happen, you can use the following equation to reach the right answer: . Answer choice E.

At the Economist GMAT Tutor, we call this the **Forbidden Method**: subtracting the ‘forbidden’ or unwanted probabilities from the total probability, which is 1.

Often the GMAT test makers word the question in a way that makes it more difficult to answer. If you can reword the question more simply, your life becomes easier!

*This post appeared first on the **Economist GMAT Tutor** blog.*