Question 1083321
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Your equation is 

{{{4/(4+r)}}} = {{{2/5}}}

where "r" is an unknown number of red balls.


Simplify and solve for "r":

4*5 = 2*(4+r)  ---->  20 = 8 + 2r  ---->  2r = 20-8  ---->  2r = 12  ---->  r = {{{12/2}}} = 6.


<U>Answer</U>.  6 red balls.
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