[tex]\text{Folosim o regula de trei simpla:}\\
5\text{ zile}.......12\text{ tractoare}\\
x\text{ zile}.......15 \text{ tractoare}\\
\text{Evident marimile sunt invers proportionale,deci:}\\
\dfrac{5}{x}=\dfrac{15}{12}\Rightarrow x=\dfrac{12\cdot 5}{15}=4 \text{ zile} [/tex]