Let $f(x, y)$ denote the output of a country. If $x$ units of its resources are spent on labor and $y$ units are spent on capital, then the level curve of $f$ with equation $f(x, y)=k,$ where $k$ denotes a constant output, is an isoquant. Each point $(x, y)$ on the isoquant corresponds to a level of investment in labor, $x,$ and investment in capital, $y,$ that results in the same level of output $k$. Suppose the output of a certain country is
$$f(x, y)=100 x^{3 / 4} y^{1 / 4}$$
billion dollars when $x$ billion dollars are spent on labor and $y$ billion dollars are spent on capital.
a. Find the output if the country spends $\$ 81$ billion on labor and $\$ 16$ billion on capital.
b. Find an equation giving the relationship between $x$ and $y$ if the output remains constant at the level found in part (a).
c. Complete the following table for the equation found in part (b).
$$\begin{array}{llllll}
x & 50 & 60 & 70 & 80 & 90 \\
\hline y & & & & \\
\hline
\end{array}$$
d. Use the results of part $(\underline{c})$ to sketch the isoquant of $f$ corresponding to the constant output found in part (b).