So we need to figure out the force equation since

(Force times distance)

Since pounds are a force let find the equation

When the monkey starts on the ground the only weight it carries is itself (P for pounds)

When the monkey is at the top he will be holding 25 ft of chain and each foot weight 1/2 pound so we get

So the slope of this line is

So the work in the integral of Force (pounds ) over distance