GMAT Probability, Simplified.

by on July 18th, 2015

Hand writing algebra equationsProbability 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 3/5. The probability that the right oar works is also 3/5. What is the probability that you can still row the canoe?

A) 9/25

B) 10/25

C) 6/10

D) 2/3

E) 21/25

1. The wrong way

The temptation is to multiply the two probabilities given to reach the answer 9/25. 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 {2/5}*{2/5} = 4/25.

Using the idea that the probability of something happening is 1- the probability that it doesn’t happen, you can use the following equation to reach the right answer: 1 - 4/25 = 21/25. 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.

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