a+b = [tex] \frac{6}{15} [/tex]
b+c = [tex] \frac{4}{15} [/tex]
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a+2b+c = [tex] \frac{6}{15} [/tex] + [tex] \frac{4}{15} [/tex]
a+2b+c = [tex] \frac{10}{15} [/tex] (daca simplificam cu 5 obtinem :
a+2b+c = [tex] \frac{2}{3} [/tex]
