A bag contains a total of 14 batteries, of which five are defective. Selecting two at random, without replacement, determine the probability that none of the batteries you select are good.

1/2
1/7
10/91
5/14

Question

A bag contains a total of 14 batteries, of which five are defective.  Selecting two at random, without replacement, determine the probability that none of the batteries you select are good.
  
       1/2
       1/7 
       10/91 
       5/14

phi · Answer

what is the probability the first battery you pick is defective?

anonymous · Answer

5/14

phi · Answer

without replacement means you keep the (dead) battery out.  How many batteries are in the bag?  how many dead batteries are in the bag (assuming you picked a bad one the first time)?

anonymous · Answer

1/7

phi · Answer

after you pick a bad battery out of the bag, how many batteries remain in the bag?
how bad batteries are in the bag?

anonymous · Answer

5

anonymous · Answer

13 rmain

phi · Answer

13 remain.  but only 4 are bad (because we are  assuming you pulled a bad one out the first pick)
now what is the probability of picking another bad battery out of a bag with 13 batteries, 4 of them bad?

anonymous · Answer

3/12=1/4

phi · Answer

now what is the probability of picking another bad battery out of a bag with 13 batteries, 4 of them bad?

phi · Answer

It is the same problem as
 14 batteries, of which 5 are defective.

except on the 2nd pick you have
13 batteries, of which 4 are bad

anonymous · Answer

so what would it be then, im confused?

phi · Answer

If you are confused, start at the beginning.

what is the chance of picking a bad battery from a bag wih
 14 batteries, of which 5 are defective.

cwrw238 · Answer

a tree diagram may help
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