gmat prep-probability

Here you go...

P(E) = (Possible outcomes for E) / (Total possible outcomes)

Here,

We need to select 2 students, one from Junior class and one from Senior class. Also, The selected students should be a sibling pair, so,

First student from Junior Class can be selected in 60 ways (60 cos thats the total number of junior students in the sibling pair group).

Now, for each selected Junior student, the Senior student can be selected in exactly 1 way, because each pair is already defined. So for every Junior student selected, only one corresponding senior student can be selected for our event to have a possible outcome.

So Possible outcomes for event E = 60 x 1 = 60

Now total possible outcomes = 1000 (Number of ways of selecting Junior Student) x 800 (Number of ways of selecting Senior Student) = 800,000

P(E) = 60 / 800,000 = 3/40,000.

hence ans (A)

HTH

Another Approach:


Prob of selecting a sibling pair from Seniors = 60/800

Prob of selecting a sibling pair from Junior = 1/1000 (since there is only 1 sibling in the Juniors that would make a sibling pair from any one of the 60 siblings in the Seniors)

P(both) = 60/800 * 1/1000 = 3/40000

(OR)

Prob of selecting a sibling pair from Junior = 60/1000

Prob of selecting a sibling pair from Junior = 1/800 (since there is only 1 sibling in the Seniors that would make a sibling pair from any one of the 60 siblings in the Juniors )

P(both) = 60/1000 * 1/800 = 3/40000

cramya wrote:Another Approach:


Prob of selecting a sibling pair from Seniors = 60/800

Prob of selecting a sibling pair from Junior = 1/1000 (since there is only 1 sibling in the Juniors that would make a sibling pair from any one of the 60 siblings in the Seniors)

P(both) = 60/800 * 1/1000 = 3/40000

(OR)

Prob of selecting a sibling pair from Junior = 60/1000

Prob of selecting a sibling pair from Junior = 1/800 (since there is only 1 sibling in the Seniors that would make a sibling pair from any one of the 60 siblings in the Juniors )

P(both) = 60/1000 * 1/800 = 3/40000

nicely put buddy

cramya wrote:Another Approach:


Prob of selecting a sibling pair from Seniors = 60/800

Prob of selecting a sibling pair from Junior = 1/1000 (since there is only 1 sibling in the Juniors that would make a sibling pair from any one of the 60 siblings in the Seniors)

P(both) = 60/800 * 1/1000 = 3/40000

(OR)

Prob of selecting a sibling pair from Junior = 60/1000

Prob of selecting a sibling pair from Junior = 1/800 (since there is only 1 sibling in the Seniors that would make a sibling pair from any one of the 60 siblings in the Juniors )

P(both) = 60/1000 * 1/800 = 3/40000

Yep, that was cool.