Question 1196711
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Urn 1 contains 7 red balls and 4 black balls. 
Urn 2 contains 4 red balls and 1 black ball. 
Urn 3 contains 5 red balls and 3 black balls. 
If an urn is selected at random and a ball is drawn, find the probability that it will be red.
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            I came to fix incorrect writing  (typos ?)  in the post by @math_helper.



<pre>
    P(red) = {{{(1/3)*(7/11)}}} + {{{(1/3)*(4/5)}}} + {{{(1/3)*(5/8)}}} = 


           = {{{(1/3)*(7/11 + 4/5 + 5/8)}}} = {{{(1/3)*((7*5*8 + 4*11*8 + 5*11*5)/(11*5*8))}}} = 


           = {{{(1/3)*(907/440)}}} = {{{907/1320}}}.    <U>ANSWER</U>
</pre>

Solved.