A certain junior class has 1000 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 2 students selected will be sibling pair
1) 3/40,000
2)1/3,600
3)9/2,000
4)1/60
5)1/15
P(sibling pair) = (total number of sibling pairs)/(total number of possible pairs).
Total number of possible pairs:
There are 1000 juniors and 800 seniors.
Total number of ways to combine 1 junior with 1 senior = 1000*800 = 800,000.
Total number of sibling pairs = 60.
Thus:
P(sibling pair) = 60/800,000 = 3/40,000.
The correct answer is
A.
Alternate approach:
Junior class:
P(picking a member of a sibling pair) = 60/1000. (Of the 1000 juniors, 60 belong to a sibling pair.)
Senior class:
P(picking the selected junior's sibling) = 1/800. (Of the 800 seniors, 1 is the selected junior's sibling).
Since we want both events to happen, we MULTIPLY the probabilities:
(60/1000) * (1/800) = 60/800000 = 3/40000.
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