# Solving linear programming problems

Find the smallest value of the function $f=21x+14y$ considering only those values of $x$ and $y$ that satisfy the constraints

\begin{eqnarray*} 15x + 22.5y &\geq& 90, \\ 810x + 270y &\geq& 1620, \\ x/9 + y/3 &\geq& 1, \\ x, y &\geq& 0 \end{eqnarray*}