15.
[tex]\it Zilnic,\ vaca \ va\ consuma\ \dfrac{1}{20}\ din\ cantitatea\ de\ hran\breve a,\ iar\ vi\c{\it t}elul \ va\ consuma\ \dfrac{1}{40}[/tex]
[tex]\it Not\breve am \ \ x\ -\ num\breve a rul\ de\ zile\ cerut.\\ \\ \dfrac{1}{40}x+\dfrac{1}{20}(x+5)=1\Big|_{\cdot40} \Rightarrow x+2x+10=40 \Rightarrow 3x=30 \Rightarrow x=10[/tex]
