5,400 is the wrong answer

The amount of the average investment for a proposed investment of $60,000 in a fixed asset, with a useful life of four years, straight-line depreciation, no residual value, and an expected total net income of $21,600 for the 4 years, is: \begin{tabular}{l} $10,800 \\ $21,600 \\ \hline$5,400 \\ \hline$30,000 \end{tabular}