se poate aplica regula dreptunghiului , pt o rezolvare rapida
c1=20%----------------> 40-x parti sol. 20%
c.f.= x%
c2=40%----------------> x-20 parti sol. 40%
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=> (40-x)/(x-20) = 1/4
=> 160-4x = x-20
=> x = 180/5 = 36%