A spring has a natural length of 18 inches and a force of 20 lbs is required to stretch and hold the spring to a length of 24 inches. What is the work required in ft-lbs to stretch the spring from a length of 21 inches to a length of 26 inches?

Question

$\begin{array}{1 1}7.6389ft-lbs\\6.6389ft-lbs\\5.6389ft-lbs\\4.6389ft-lbs\end{array} $