so we need to find .
there is a constant force of 100lb being hauled.
there is also a variable force at play, that is, the weight of the chain. as we lift some of the chain, the force decreases, as we have to lift less weight. the chain weighs 3lb/ft, so for every foot we pull, we have 3lb less force to haul.
let x be the distance we already hauled it