Question 1195011
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In an urn there are 3 white and 4 black marbles. They are taken out of the urn one by one. 
What is the chance of the first and last marble being white?
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In all, there are 3+4 = 7 marbles.


There are  {{{7!/(3!*4!)}}} = 35 possible DISTINGUISHABLE permutations of these marbles. 
It is the number of all possible distinguishable outcomes of the experiment.


Of these 35 distinguishable permutations, there are 5 distinguishable permutations, 
where white marbles are in the 1-st and in the last, 7-th position, and in some 
position between 2-nd and 6-th for the remaining third white marble.


Therefore, the sough probability is  P = {{{5/35}}} = {{{1/7}}}.    <U>ANSWER</U>
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Solved.