A 5.0-meter-long plank rests upon a fulcrum (pivot/midpoint). On one end of the plank, there is a 4.0 kg backpack, while on the opposite end, there is a 1.5 kg lunchbox. In order to maintain equilibrium on the plank, determine the distance at which a 2.5 kg rat should be positioned to the left of the plank's midpoint.