A passenger railroad car has a total of 8 wheels. Springs on each wheel compress—slightly—when the car is loaded. Ratings for the car give the stiffness per wheel (the spring constant, treating the entire spring assembly as a single spring) as 2.8 × 10⁷ N/m. When 30 passengers, each with average mass 80 kg, board the car, how much does the car move down on its spring suspension? Assume that each wheel supports 1/8 the weight of the car.