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abcgmat
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1. 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
[spoiler]OA:A[/spoiler]
60/1000 * 1 /800 = 3/40,000
here we are not considering the situation of selecting one of the 60 from 800 students and 1 out of 1000 students: 60/800 * 1/1000
2. A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously
from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.
[spoiler]OA: D[/spoiler]
For option B we are considering two scenarios
D*(notD) + (non) D * D = n/10*(10-n)/9 + 10-n/10 * n/9
When do we need to consider only 1 scenario and when do we need to consider two scenarios
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
[spoiler]OA:A[/spoiler]
60/1000 * 1 /800 = 3/40,000
here we are not considering the situation of selecting one of the 60 from 800 students and 1 out of 1000 students: 60/800 * 1/1000
2. A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously
from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.
[spoiler]OA: D[/spoiler]
For option B we are considering two scenarios
D*(notD) + (non) D * D = n/10*(10-n)/9 + 10-n/10 * n/9
When do we need to consider only 1 scenario and when do we need to consider two scenarios
