GMAT Probability, Simplified.
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?
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.