A certain junior class has 1,000 students and a certain senior class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?
A) 3/40,000
B) 1/3,600
C)9/2,000
D)1/60
E) 1/15
I have difficulties with probability. I have gone back thru the Manhattan GMAT guides to no avail. Any approaches from anyone? Without doing much math, is there a way to look thru the answer choices and determine what is clearly incorrect?
For this problem, i look at it like this: there are 120 students who can be paired. 120 out of 1800 students times 119 out of 1799.....correct in my thinking?
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3 postsPage 1 of 1
- SHAFTBMF
- GMATAttack
Response...
This is an "AND" probability question because both individuals must be a part of the sibling pair for the winning outcome to occur.
Since this is an "AND" question, it will involve multiplication. Also keep in mind that 60 "sibling pairs" is really 120 people.
Probability of selecting one sibling pair from class of juniors =
60 / 1,000
Probability of selecting a sibling pair from the senior class that is THE match to the one you selected from the from the junior class? There is only one person that would be the match, so winning outcomes / total possibilities =
1/800
Therefore: Answer = 60/1000 * 1/800 = 60/800,000 = 6/80,000 = 3/40,000 (A)
Since this is an "AND" question, it will involve multiplication. Also keep in mind that 60 "sibling pairs" is really 120 people.
Probability of selecting one sibling pair from class of juniors =
60 / 1,000
Probability of selecting a sibling pair from the senior class that is THE match to the one you selected from the from the junior class? There is only one person that would be the match, so winning outcomes / total possibilities =
1/800
Therefore: Answer = 60/1000 * 1/800 = 60/800,000 = 6/80,000 = 3/40,000 (A)
- StaceyKoprince
- ManhattanGMAT Staff
- Posts: 9349
- Joined: Wed Oct 19, 2005 9:05 am
- Location: Montreal
GMATAttack has it.
Set up problems like this sequentially - what's the first thing you do? Pick one person (either a junior or a senior - your choice). Then you want "AND" the sibling, which means multiplication, so write down that x sign. Then pick the second person (whatever you didn't pick the first time) - and just remember, as GMATAttack notes, that once you've picked the first person, you have a major constraint for the second person - only one person is going to be the first person's sibling.
Note: simplify fractions before multiplying - save yourself some time. :)
Set up problems like this sequentially - what's the first thing you do? Pick one person (either a junior or a senior - your choice). Then you want "AND" the sibling, which means multiplication, so write down that x sign. Then pick the second person (whatever you didn't pick the first time) - and just remember, as GMATAttack notes, that once you've picked the first person, you have a major constraint for the second person - only one person is going to be the first person's sibling.
Note: simplify fractions before multiplying - save yourself some time. :)
Stacey Koprince
Instructor
Director, Content & Curriculum
ManhattanPrep
Instructor
Director, Content & Curriculum
ManhattanPrep
3 posts Page 1 of 1