Question 491062
A bag contains a total of 20 batteries, of which four are defective.  Selecting two at random, without replacement, determine the probability that none of the batteries you select are good.
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The sample set is the ways to choose any 2 out of the 20, or 


These:

16 good ones, choose 2, or 16C2, 


out of these:


20 batteries choose 2 or 20C2

Answer: {{{(16C2)/(20C2)= ((16*15)/(2*1))/((20*19)/(2*1)) = ((16*15)/(2*1))*((2*1)/(20*19)) = ((16*15)/(cross(2*1)))*((cross(2*1))/(20*19)) = (16*15)/(20*19)=240/380=24/38=12/19}}}

Edwin</pre>