$\"\"$

The value of a tract of timber is $\"\"$ , where $\"\"$ is time in years.

$\"\"$ corresponding to $\"\"$ .

Money earns interest continuously at $\"\"$ .

The present value of the timber at any time $\"\"$ is $\"\"$ .

Substitute $\"\"$ in $\"\"$ .

$\"\"$

Differentiate the above equation with respect to $\"\"$ .

$\"\"$

Equate $\"\"$ to zero to find the year in which the timber is harvasted to maximum profit.

$\"\"$

$\"\"$ , $\"\"$

$\"\"$

Therefore, the year in which the timber is harvasted to maximum profit is $\"\"$ .

$\"\"$

The year in which the timber is harvasted to maximum profit is $\"\"$ .