You may have multiplied 5 yards by $\displaystyle\frac{{{12}\ \text{ feet}}}{{{1}\ \text{ yard}}}\usepackage{cmbright}\Huge\$ and then added 1 foot. You need to multiply 5 yards by $\displaystyle\frac{{{3}\ \text{ feet}}}{{{1}\ \text{ yard}}}\usepackage{cmbright}\Huge\$ and then add 1 foot. The correct answer is 16 feet.