|
|
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.
Customized
for You
Track
Your Progress
Practice
Pays
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
16 Jun 2020, 07:36
Kudos
Bookmarks
varotkorn
If you delete those words, then it's not clear what the question is asking. Do you want the probability he gets the phone number right precisely on his second attempt? Or in one of his first two attempts? Here we have 25 possibilities, one of which is right. If he'll pick two of the numbers, he'll have a 2/25 chance to pick the right one. Mathematically, the problem is essentially identical to a different question you asked about yesterday, which was something like this - if you have 25 employees, including Bob, and randomly pick a President and Vice-President, what is the probability Bob will be President or VP? The answer is 2/25.
Some solutions above produce the answer 49/625, which is not correct, unless he might pick the same phone number twice. You can see why that method is wrong just by imagining there are only two possible phone numbers. Then in two attempts Bob should have a 100% chance of getting the number right, but if you use the method that produces the answer 49/625, you would get an answer less than 100% to the two phone number question.
I have no idea why, in the original question, they appear to be asking for an estimate, nor why the fractions in the answer choices are not reduced.
16 Jun 2020, 07:47
Kudos
Bookmarks
IanStewart
However, IMHO if the question were the probability that he will be able to find the correct number in
Moreover, as I learned from you yesterday, those 2 attempts can be from the tenth and eleventh attempts (not necessarily the first 2) and the result is still the same
IanStewart
Totally agree here! Some imprecise reply above is actually of expert in quant
That's why I really need your view on this
16 Jun 2020, 07:57
Kudos
Bookmarks
varotkorn
He presumably stops guessing when he finds the right number. So he won't make two attempts half the time, when he guesses right the first time. That's why the question says "at most two attempts", rather than "in two attempts".
Well-written math questions often use some language that might, at first glance, appear unnecessary, because sometimes those extra words ensure the question isn't open to two different legitimate interpretations. The wording they use here is necessary, so no one can interpret the question to mean "on precisely his second attempt".
