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18 May 2021, 04:15
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E
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
25%
(medium)
Question Stats:
81% (01:18) correct
19%
(01:16)
wrong
based on 73
sessions
History
Date
Time
Result
Not Attempted Yet
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
A. 9/25
B. 10/25
C. 6/10
D. 2/3
E. 21/25
18 May 2021, 04:15
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Bunuel
Official Explanation
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 x 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.
18 May 2021, 06:35
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P(Left) = \(\frac{3}{5}\)
P(Left') = \(\frac{2}{5}\)
P(Right) = \(\frac{3}{5}\)
P(Right') = \(\frac{2}{5}\)
P(can row) = 1 - cannot row = 1 - (\(\frac{2}{5} * \frac{2}{ 5} = \frac{4}{25}\)) = \(\frac{21}{25}\)
Answer E
P(Left') = \(\frac{2}{5}\)
P(Right) = \(\frac{3}{5}\)
P(Right') = \(\frac{2}{5}\)
P(can row) = 1 - cannot row = 1 - (\(\frac{2}{5} * \frac{2}{ 5} = \frac{4}{25}\)) = \(\frac{21}{25}\)
Answer E
